I'm starting a minecraft server and i was wondering?

Not really.
All you have to do is make an agreement stating what exactly will be given and to make sure the agreement is very specific and detailed to whatever you would be selling, make sure there's also like a strict refund time/no refund etc…

Bottom line, make a detailed agreement.

Specify what the donations will be used for (server maintenance, donations to mod creators, after game beer party, whatever) and you should be fine as long as the income isn't large enough to be considered for taxes, and it usually isn't.

And if someone wants to sue you for their $20 back just send them their $20 back.