Learning repertoire is another life-long (and fascinating) part of being a conductor.
For the beginning conductor it seems a daunting task, so first know that it's never-ending. Don't worry that you don't yet know enough, just get started.
First, of course, you'll learn the repertoire you sing. That doesn't mean it will be appropriate for the first ensembles you conduct, but it's an important building block in your knowledge. That said, take every opportunity you have to sing in more ensembles. As an undergraduate at the University of Washington when Rod Eichenberger led the choral program, I sang in almost every grad student's recital choir. In that way I was exposed to much more repertoire than I would have just singing in the UW Chorale. I also occasionally sang with other choirs, mostly to see a particular conductor work, but I also learned a huge amount of new repertoire.
I should also say that you need to start building your own score library. This is tough (financially) for a student and perhaps even for the young conductor starting her career, but will pay long-term dividends. In one of my methods classes we did a lit project writing down information about repertoire on file cards (yes, it'd be on computer today!). While this is helpful, later when you need to decide on one piece or another for your choir, you need to look at the specific score itself: is the tessitura too high for your tenors? range too low for your basses? how difficult is it to learn? So buying scores needs to be some sort of budgetary priority.
Go to concerts. If the symphony nearby is doing a choral/orchestral work you don't know--go! If another college choir comes to town on tour--go! If there's a good church choir doing a concert--go! Just go and listen and learn. Take notes in the program if you love particular pieces.
I was also lucky at the UW in that I started hanging around Rod's office listening to his discussions with the grad students. This was in the days when publishers gave away many more comp copies of scores than they can afford to today, and Rod had a couple huge stacks that hadn't been filed. Since I was hanging around, he told me if I'd file those scores for him I could keep any duplicates. This was a double win: I got the beginnings of a very good choral library (even things like some of the Bach motets), but would also look through each file when I put a piece away (they were filed alphabetically by composer). If I put away a piece by Hindemith, I'd look to see what else Hindemith had written. Consequently I got a great overview of the repertoire.
When I was in Europe on a choir tour with the UW Chorale I sought out music stores there. I still have Doblinger scores that I bought in Vienna and a few I bought in Paris. In Paris my French pronunciation clearly wasn't too good since I asked for "choeur" music and they brought me to the files of "cor" (horn) music! After that tour I stayed in Europe for another couple months and picked up music in other places as well.
Conferences are another great place to get to know repertoire: first, you'll hear lots of choirs singing great music you don't know. Keep notes of pieces you particularly like so you can acquire the scores later. Or if you hear a particular composer whose music you like, make a note to find out what else they wrote. And, of course, spend time in the exhibits, both of music stores and publishers. Between the many ACDA, MENC, IFCM, ACCC and other conferences I've attended, I've been exposed to a huge amount of repertoire. But you have to take notes or buy scores to remember what was worthwhile!
Workshops/Festivals are much the same. I was a student at the Oregon Bach Festival very early on (1972, the third year of the Festival, when it was still relatively small) and Helmuth Rilling told us if we'd give him our addresses, he'd pass them on to Günter Graulich, then the chief editor at Hänssler and after owner of Carus-Verlag. So I started getting their newsletters at that time, first in print, than later via email, and always was kept abreast of the huge catalogue that's been built over the years (and I still get their email newsletters today). Since I was particularly interested in early music, it was a great resource. Through Bob Scandrett I later got to know Graulich when he did a workshop at Western Washington University in Bellingham, along with Greg Smith and Louis Halsey. Bob then arranged a study tour of England in 1975 (follow the link from Bob's name to a series of blog posts about that trip) which introduced a huge number of composers and their music to me. This wasn't all new music--I heard Roger Norrington's Heinrich Schütz Choir rehearsing Monteverdi's 1610 Vespers which inspired me to learn the music and find a way to program it a year later.
Recordings are another obvious resource. With something like Naxos available today, you can have access to an amazing library of recordings online--many more than you'll ever be able to listen to. When you hear music of a composer you like, it can be a quick way to explore some of their other repertoire. And as a "dinosaur" of sorts, I still buy a fair number of CDs--old habits die hard!
As you begin to conduct your own choirs, you'll have reason to explore specific repertoires. When I taught at Mt. Holyoke I had reason to learn a lot of women's choral music. The wonderful library there provided a great resource, of course, but I also looked outside for other repertoire. During my time at PLU we started a men's chorus which I conducted, and that was the impetus for looking for male chorus repertoire. Since I haven't conducted a women's or men's chorus since then, I haven't kept up with the field, but whether you're looking for repertoire for the male changing voice, madrigal repertoire, choral/orchestra rep suitable for your church or HS choir, etc., you'll explore those areas in much more depth. A couple years ago I took on an Interim Choirmaster position at a big Episcopal Church in Dallas, where they sing Evensong every Sunday during the academic year. That gave me the excuse to learn a huge number of "Mags & Nuncs," along with Preces and Responses and much other Anglican repertoire I love. Whether it's one of the genres I've mentioned or vocal jazz, spirituals, gospel, African Freedom songs, or music from other cultures, there's always something new to discover, depending on the needs of your choirs.
Your colleagues will also be a fantastic resource once you become a conductor. You need to ask them what they consider the core repertoire for the middle school, or high school training choir that you're now conducting. Or to say, "What have you done recently that was a huge hit with your singers?" I've found my colleagues over the years to be incredibly generous. When I first became conductor of Pro Coro Canada in Edmonton, I traveled to Vancouver, B.C. and spent the day at Jon Washburn's house, looking through his vast library of music by Canadian composers and asking questions. I think you'll find similar generousity--just remember to pay it back to other conductors one day!.
If you have further ideas, please share them in comments!
While learning our vast repertoire is challenging (too vast to learn more than a fraction), it's one of the joys of being a conductor--there's always a new composer or piece to discover and then to share with your singers and audience! Don't be afraid of how much you have to learn, just begin.