cleaning up a donation bike

[laura*]

... an old huffy ... 18 speed

Almost certainly 3x6.

How do I know what size chain to get?

Today, five, six, seven, and eight speed cassettes all use the same chain. So ask the shop for an eight speed chain. (Actually, there are some older chain designs that are rated for fewer speeds. KMC makes one that is good for a max of 7 speeds - when we stocked it at the co-op, it sold for $6. This chain would work for you!)

You'll also of course have to trim the new chain to the correct (same as the old one) length...