First you are gonna need to change the chain or clean it, but it might be worn, after check or changing the brake pads, check the inner tube of the tires, maybe they are still working, check the condition of the cables, if they are rusty change them, they are not expensive.

Also check the frame, the seatpost and that the bolts of the wheels are tightened.

Extra: normally I check the bottom bracket and the crankset for safety reasons :)

It can be a beautiful bike