How to fix an old bike I found?
If you don't want to spend any money, just take it slowly at first to ensure that the brakes and steering are working properly.  Try twisting the bars while holding the front wheel between your legs to make sure the bars don't turn with a decent amount of force. Ensure that the wheels spin freely and don't rub on anything. It's ok if they wobble a bit on such an old bike, but if there is excessive wheel wobble you might want to have a professional look at it. 

Alternatively, you can take it in to your local bike shop for a tune up and safety check. Looks like a single speed with a coaster brake so there really isn't much to go wrong or adjust but you might be best off having a professional take a look at it and making sure things are in working order.