First thing is look at it. Is there any rust on the chain, and cables? Is there rust on the derailleur's (gear shifetr's**? When you squeeze the tires are there cracks in the sidewalls or the top surface? Then check for loose parts, cracks, bends etc. If the cables are rusted replace them and the housing. If the tires are dry rotted replace them. If the chain is rusted replace it.**You may need to replace the freewheel or cassette as well, since chains wear with your rear cogs**.
If any of this is beyond your expertise then I highly reccomend a visit to your local bike shop.