If you can imagine having any proprietary interest in the project --- some line of business or donation revenue stream built on your thing that will naturally go to you --- you probably want to GPL. GPL adds use restrictions that make it difficult (though not impossible) to commercialize open source software, and those restrictions will apply only to your downstream users (you are not bound by the GPL for code you own and can relicense it as you see fit). This is part of the business model for a lot of open source companies.
If you want to go really nuts with that, look at AGPL, which imposes even more restrictions. People are generally cool about GPL and less cool about AGPL.
If you don't care about any of that, MIT license is the most normal choice.