Go to a Sleep Country store. They have a boatload of mattresses set up for you to try, from the extra cheapo's to the $5000. Sleep around.
See what you like. Don't let the salesperson get pushy. You'd be surprised how many different sorts of mattresses there are, and how they feel differently from each other. Once you've done that, think about a price range, and look around for something you like in that range.