hi every one new to cycling bought a used trek 830 for $15 i do not know where to start the bike has rust on the chain and when u back pedel the chain get loose the gear shifting is very noisy and sloppy how do you clean rust off off the chain and some part of the wheels
I would just get a new chain. It looks like a 7-speed there and a chain for those are around $10.
I would take apart the cassette and use some naval jelly to get rid of the rust. Or just buy a new cassette, here is one for $12
I realize you only bought the bike for $15 but it is just going to take too long to get that rust off to save a few dollars.