I got a free snake... I don't know how to take care of it and it looks weird

[Kitty]

Me and my roommate went to go buy a 20 gallon aquarium to put our beta fish into. The guy who sold it to us offered to give it to us for free if we took the animal that he was keeping in it. He told us that it was a girl Rosy snake and it's two years old, but we didn't ask anything else because we didnt really want to keep it. We tried to take her to the only pet shop that's close to us (and it's a 30 minute drive) but the owner told us that he was full until he could sell another snake. We kept her the last two days in the aquarium and we're kind of rethinking our decision to sell her. The problem is that we don't know how to take care of a snake and it looks kind of weird. It's kind of saggy with some loose skin and it's face skin is stuck on it. It kept on running around everywhere until we went to go buy a snake house that looks like a cave. Now it just stays in there and doesn't go crazy unless we take its house out. We put a little cat water bowl in there but she just kept knocking it all over the place so we had to go all the way back to get one that's for snakes. It's huge and covers probably 25% of the aquarium. We also made it a bed inside the cave with a shirt, but now we don't know what else to buy it. The fish tank looks kind of small but it never comes out anyways so we don't know if it even needs more room. Also, how are we supposed to know when to feed it? I know that they eat rats and mice, but I don't know which one this snake eats or when I should give it one next.

Ummm, I think that's all the questions we had, but any advice is good.