Help buying mattress

Im looking at getting a new mattress and wanted some advice on buying. Today im gonna go look at mattresses and get an idea of cost and what i like. I do have a few questions off the top of my head.
Should i get a new box spring to go with it?
What should i look for when buying a new mattress?
Any good brands recomended?
Thanks for any help you can give me.
i found that the mark up is insane on that shit. I have a friend that works at a furniture store and is able to buy at almost 50% off (can buy at 10% over cost) now i know to wheel and deal on any furniture item ESPECIALLY when it’s time to throw out the old mattress.