Title says it all. When saying それは椅子 would I use あります or です? I've been using です, but from what I've read, it's used to describe an object, and あります to declare it's existence. So I'm thinking あります would be the correct verb to use.
それは椅子です = That is a chair. (as in "that thing is a chair") 椅子があります = There is a chair. (as in "a chair exists" somewhere)
When you use それは~です you can think of it like saying "that's a ~". In the case you mention you should use です.
あります is the equivalent of there is/are and to have, depending on the context.
it could actually be either, depending on the context.
「それは椅子です」 - that's a chair / that would be a chair
「何？椅子ないかってー？そりゃ、椅子ありますよ」 - What? If we have chairs? Why, of course we have chairs.