I'm having a hard time finding an appropriate replacement chain for my bike. Its a 21 speed, Wal-mart brand mountain bike. My local bike store has chains that were up to a 10 speed. I know I need a chain tool, and to measure the new chain using the old chain, but I'm unsure what type of chain to buy.

Thank you!

PS I'm in the market for a new bike, as I currently commute to and from work on my bike. I've been using the cruddy Wal-mart bike because it was a gift. As for now, I want to tune it up and make sure it can survive another couple months of riding! It got rather rusted this winter when I stored it in my apartment building's basement.