Getting the right Bottom Bracket
If you have an idea of what model of frame it is, it would help. Otherwise, your best bet is to take it to a bike shop and try an ISO bottom bracket (gently) to see if it fits, or measure the threads and their orientation to match to this chart. 

There are also universal threadless BB's like this one from Velo-Orange which skirt the issue by expanding to fit. In this case, the thread condition doesn't matter and you can reuse the threads for the proper bottom bracket provided they're in good enough condition. These are a good bit more expensive than your typical Shimano ISO bottom bracket, though. 

The spindle length needs to be selected in conjunction with the crankset to achieve the appropriate chainline. I'd suggest going to a shop to pick up a proper bb and crankset at once.