I've been biking for 3 years now and I've been using a cheap wal-mart mountain bike. I want to upgrade my bike to a better brand with a price range of $300-$800. I am not very familiar with any bikes that are good. Do you have any recommendations?

Some to look at.

https://websterbicycle.com/itemlist.cfm?category=48

You havent said what sort of riding you want to do. 10 Wheels pointed you to a decent list for sporty riding, but for utility riding these might be more suitable https://websterbicycle.com/itemlist.cfm?category=53

i wanna use it for exercise purposes or possibly commuting. i also live in a hilly place so is a hybrid bike the way to go?

**********

You should get an entry-level road bike w/drop bars. Giant Defy 3, Felt F100, Fuji Newest 3.0 etc. They're generally much lighter and stiffer than hybrids, which will help in the hills. They're also more aerodynamic in windy conditions.

I don't share many people's distaste for hybrids. For all-around riding, they work fine. The pound or two of extra weight over a "real" road bike is insignificant. They're not ideal for any single use, but many cyclists, including me, don't do just one type of riding. My Atlantis is sold as an "all around bike," really a disguised hybrid, and I've used it for everything from fire trails to centuries with just a tire swap.
I think you should go to a bike shop--a real one--and tell them your budget and what kind of riding you want to do. If they tell you they don't have anything, try another shop.

oh cool. i was actually thinking of going to my LBS. thanks very much!