That's ok, I still have plenty of silly questions The cassette is the cluster of cogs on the rear wheel. A larger cog will spin easier but move you a shorter distance for every turn of the pedals. The size is usually marked on the cog itself, if not you could always count the teeth. I had the parts lying around to cobble it together when my RD went out, but you can find low-end but god parts for cheap.

Ultimately, though, the best way to get better at climbing is... to climb