Looking to upgrade my stock speakers. What do I need to know before I go to the local audio shop?

I have a 2013 Focus SE. I want to replace the stock speakers and add in a single sub. I'm just curious what I need to know so I'm not clueless going to the store.

I'm not looking for anything crazy. I just want to be able feel that thumping bass and have better sounding highs.

Also how much do you think I should expect to spend? I'd like to keep it around $2000 if possible.