Hi ancientcampus, I'm in pretty much the same boat as you: same situation, same objectives. I'm not sure if you've heard of it, but I found the advice at www.givingwhatwecan.org really helpful. They have recommendations of the best charities, a supportive community, and advice for students.

What they suggest - and what I'm doing - is to not donate a substantial amount right now, but instead to wait until you're earning. However to get into/keep the habit of donating, a donation of 1% of your spending money is suggested.

As to whether you should wait to donate until after you have paid off your student loans, that would depend on what country you live in.