I think you're maybe misunderstanding something here. The わ you're thinking about is just a normal katakana, a syllable. On the other hand, the は that you're talking about is a grammatical element. When は is used by itself, i.e., it's not a part of some word, its pronunciation changes to "wa". It is still written as は, not わ. I don't want to complicate things further by explaining the grammar usage of わ, since that would make you even more confused. I suggest you pick up a textbook, like Genki and learn about は. It's one of the first things explained there.