bikes are pretty simple vehicles-a petal crank, a couple of gears and a chain. Obviously you want to look for serious rust on the metal parts, also tighten up all the fittings, checking for stripped out ones. Oil the crank and chain. Check the chain for binding between links -if that occurs, then remove the chain and soak it in a pan of cleaner and try relubing it, if it is still binding, replace it. That is pretty much it, unless you want to go deeper and start looking at steering stem bearings and crank bearings.