I got a Specialized Hardrock for really cheap but it’s in worse wear than I first thought. The front chainring teeth are bent and I need a new rear derailleur. The cassette on the rear tire seems fine. Its 3x7 and I dont really understand what size of front chainrings to buy or how to make sure they’re compatible with the crank and the rear cassette. Also the the front derailleur is a shimano SIS and im not sure if theres a specific rear derailleur that I need to make sure it’s all compatible.
Im looking at used parts and really just looking for direction to get it up and running if anyone has any advice or can point me to some good info online. I was thinking of taking it to a bike shop but I would prefer to learn how to do things myself