The Shaliach Tzibbur is the person who leads the congregation (tzibur) in prayers and at times fulfills its obligation on its behalf.
Who to appoint as Shaliach Tzibur
- Any male Jew is fit for being a Shliach Tzibbur as long as the congregation accepts him. 
- In general factors that are important for a Shaliach Tzibbur include: free from sin, good reputation, humble, accepted by the congregation, has a good voice, and is familiar with Tanach.
- Some poskim say that someone who has a different havarah than the shul should adopt the havarah of the shul when he davens there unless it is a havarah that wasn't accepted by the geoldim or it is too difficult for him. Others say it is acceptable to use his own havarah even though it is the official one of the shul.
- If someone can't distinguish between the pronunciation of Aleph's and Ayin's, Chet's and Hey's, or Sin and Shin's he shouldn't be a shaliach tzibbur unless the entire congregation speaks like that.
- Sephardim should not serve as a Shaliach Tzibbur of an Ashkenazic congregation unless they agree to let him pray according to his minhag.
- A sinner shouldn't be a shaliach tzibur even on occasion until he repents.
- Someone who had a bad reputation when he was a child shouldn't be appointed as a shaliach tzibbur. Some poskim say that a baal teshuva can be appointed as a shaliach tzibbur since is like a new person.
- A child should not be a shaliach tzibur. If there is a practice to have a child be shaliach tzibur for arvit there is what to rely upon.
- Someone who is 13 but doesn't have two pubic hairs can't be a shaliach tzibur to fulfill the obligation of others on a Biblical level but only for rabbinic obligations.
- An impotent can be a shaliach tzibbur if he's 20 and has signs of being impotent.
- It is permitted to have a boy that is above 13 who doesn't have a beard be a shaliach tzibur on occasion but he can't be appointed as the shaliach tzibur to daven regularly or even on certain occasions.
- In a minyan of students who don't have a beard such as in an elementary school they can appoint a shaliach tzibbur who doesn't have a beard.
- A congregation can't be mochel on their kavod to appoint a shaliach tzibur who doesn't have a beard.
- Someone above 20 even if he doesn't have a beard can be appointed as a shaliach tzibbur.
- A blind person can be a shaliach tzibbur as long as he doesn't do kriyat hatorah. Some say that he shouldn't be a shaliach tzibbur for yamim noraim.
Hard of Hearing
- A person who can't hear himself at all shouldn't be the shaliach tzibbur.
- Someone who is wearing clothing that is torn and his shoulders and arms are exposed shouldn't be a shaliach tzibbur.
Shaliach Tzibur for Yamim Noraim
- Preferably, the congregation should carefully choose a proper Shliach Tzibbur who is married, thirty years old, and the more he is learned and practices good deeds the better.  However, someone who is learned and Yireh Shamayim is preferred over someone who lacks these qualities but fits the requirements of being married and thirty years old. 
- The Shaliach Tzibur may use secular tunes for Kaddish and Kedushah, though it is preferable to use tunes that the words to which have already been forgotten.
- One should not elongate words or stop in the middle of a word, or put the stress on another syllable in order to hit every note, as it causes the words to lose their meaning and people will end up answering Amen Chatufah. Similarly, he may sing Hashem's name, as long as it is Mil'ra and not Mil'eil.
- The Shaliach Tzibbur should pray with much joy and fear for the privilege to thank Hashem as Shaliach Tzibbur.
- One may not repeat words, as it is considered a hefsek.
- Rama 581:1
- Tur 53:1. Gemara Taanit 16a lists the following qualities for someone to be a proper Shaliach Tzibbur: a person who has children but doesn't have enough sustenance, free from sin, good reputation, works hard, humble, accepted by the congregation, good voice, is familiar with the text of Tanach, and learns Midrash, Halacha, and Aggada.
- Dirshu 53:45 cites three approaches: Rav Shlomo Zalman 5:20 said that a shaliach tzibbur should try to daven with the havarah of the congregation except for havarah Sephardit in an Ashkenazi shul since that wasn't established with daat torah. But if it is too difficult he can daven in his own havarah. Rav Elyashiv (Yisa Bracha 2:21) says even if it is difficult he shouldn't be the shaliach tzibbur except from Ashrei and on. The Steipler (Orchot Rabbenu 1:50) said that he can use whichever havarah he wants.
- Shulchan Aruch O.C. 53:12, Mishna Brurah 53:37
- Yabia Omer OC 6:10:8
- The Mishpat Tzedek 2:75 writes that a wicked person (rasha) can not serve as a shaliach tzibur. He proves it from the fact that a sinner is disqualified as a shaliach tzibbur (Shulchan Aruch O.C. 53:4). Also the leniency of having a less than qualified shaliach tzibbur on occasion is only applicable to a teenager and not a wicked person unless he does teshuva. The Eliya Rabba 53:10, Olot Tamid 53:7, and Pri Megadim M"Z 53:3 codify the Mishpat Tzedek. See Bach 128:20 who seems to apply the leniency of having a unqualified shaliach tzibur on occasion more generally than a teenager.
- Tanit 24a, Tur and Shulchan Aruch 53:6
- Rav Wosner (Tzohar 2:124) cited by Dirshu 53:31
- The Mishna Megillah 24a establishes that a child shouldn't serve as a shaliach tzibur. Rashi s.v. hu explains that the reason is because he can't fulfill the obligations of the congregation. Rashba 1:239 cites the Raavad who explains that it isn't respectful to the congregation to have a child be the shaliach tzibur. Rashba agrees with Raavad. (Kol Bo 11 has another reason being that a shaliach tzibur requires an ish as it says Yishayahu 50:2 (see Brachot 6b).) The Bet Yosef 53:10 records a minhag for a child to be the shaliach tzibur on Motzei Shabbat. He says that although it isn't proper and the Avudraham and Rabbi Yitzchak De Leon protested the practice the Bet Yosef presents an explanation for it. There is what to rely upon because according to Rashi in arvit where there is no obligation it is acceptable. Also, according to the Raavad the congregation can forgo their honor. The Shulchan Aruch O.C. 53:10 writes that such a practice doesn't have to be protested. Magen Avraham 53:12 and Taz 53:5 agree. Dagul Mirvava explains that there's no difference between Motzei Shabbat and any night of the week besides Friday night when it is more problematic for a child to be shaliach tzibur since he is exempting people who forgot Mashiv Haruach with Mein Sheva.
- Mishna Brurah 53:25 citing the Pri Megadim E"A 53:10
- Shulchan Aruch O.C. 53:9, Mishna Brurah 53:20. See the Mishna Brurah who cites the Maharshal who says that it isn't honorable for the congregation to appoint an impotent as the established shaliach tzibbur.
- The Mishna Megillah 24a implies that a boy above bar mitzvah can be a shaliach tzibur. But Chullin 24b establishes that a boy can't be a shaliach tzibur unless he has a beard. Tosfot Chullin 24b s.v. nitmaleh explains that a child can be a shaliach tzibur on occasion but not appointed or for a tanit. (Tosfot Megillah 24a s.v. eino implies that it is permitted under either condition that it is on occasion or it is not a tanit.) The Rashba Chullin 24b, Ran Chullin 5b s.v. garsinan, Rosh 4:22, Kol Bo 11, Tur and Shulchan Aruch 53:6 accept the Tosfot and allow a bar mitzvah boy to be shaliach tzibur on occasion.
- Rav Shlomo Zalman (Halichot Shlomo Tefillot ch. 2 Orchot Rabbenu 76) cited by Dirshu 53:34
- The Bet Yosef 53:6 suggests that according to the Rashba and Rambam the congregation can forgo their honor but according to the Rosh it isn't possible. Bach 53:2 and Taz 53:2 vehemently argue that it is impossible. Magen Avraham 53:9 and Chayei Adam 29:10 disallow a congregation forgoing their honor initially. Mishna Brurah 53:23 rules like the Taz that the congregation can't forgo their honor.
- Shulchan Aruch 53:8
- Megillah 24, Shulchan Aruch O.C. 53:14
- Mishna Brurah 53:41
- Mishna Brurah 53:41 citing Rabbi Akiva Eiger that although he fulfills his obligation because you don't need to hear yourself bedieved but initially that is important. Also, in Biur Halacha 199:10 s.v. mitztarfin he adds that even if he isn't fulfilling anyone's obligation today the institution of a shaliach tzibbur doesn't change.
- Megillah 24a, Shulchan Aruch O.C. 53:13, Mishna Brurah 53:39
- Rama 581:1
- Mishna Brurah 581:13
- Yechave Daat 2:5 explains that the main issue with secular music is the words, so it is best not to pick a secular tune that will remind the Tzibbur of the words to the song. Rather, one should use a tune that nobody remembers the words to, since by doing so one brings the tune into the domain of kedusha.
- Yechave Daat ibid.
- Yechave Daat 2:5
- Yechave Daat 2:5