I need help with my AC. please help.?

Alright, I have a 1998 mercury Sable and I bought it over a year ago. The AC/radio didn't work. I got a new radio from a junk yard today and all the blower motors and the radio are up and running but its not throwing cold air. Do I just need to put Freon in the car? Or what do I need? I'm not sure what to do from here. I'm looking to do everything on my own. Shops charge an arm and a leg just to take a look at ur car when I feel like I'm pretty close to figuring this whole thing out. I don't know where to put the Freon or anything tho =/ lol. My friend said buy freon with dye to make sure nothing is leaking.