Need to replace my hard drive - product tips? Other tips?

So, I got to wake up this morning to a great message on my computer. “Hard drive failure is imminent, please back everything up”. So, now that I’ve backed everything up (I think hope), I need to buy a hard drive. But which one? By whom? And how do I get my Windows 7 back onto it? And how do I install it?

It’s a HP Pavillion a6230n desktop.