REDCMPG has pretty much told you what you need to do to install a head unit. Picking one will be your next task.
Do you want navigation capabilities? Smartphone integration? Got kids that sit in the back you'd like to play a movie for? Do you appreciate sound quality to the degree that you'll be looking to ditch the Monsoon amp? Consider all of that when you're looking for a head unit.
I'm kind of partial to Pioneer products. Arguably, they produce the best sound of all of the mainstream - not esoteric - head units available. And you're shopping at the right time. The new lineup should be released sometime next month. I think dollar for dollar they have the most feature rich systems available. Clarion and Kenwood also make units worthy of consideration.
If you're interested in navigation, I'd consider a unit that will let you display mapping from your smartphone. The maps there are always up to date, and you can shave almost half off the cost of the head unit. See it work before you drop your money. For all of my love of Pioneer, I am not happy with their "app mode." The 2014 models are supposed to handle that differently.
