Scratch the StarSan (for Cleaning)....Go with PBW...or Oxyclean. You need the detergent power to really scrub out years of soda residue. You will want to use the starsan to sanitize when you are finished before you fill it.
An O-ring replacement kit comes with the big one for the lid, 2 little ones for under the posts (you gotta take the posts off) and 2 medium ones for the connections on the posts. You can (and sometimes have to) replace the "poppets" which are the little valves inside the post. They just fall out most of the time, but occasionally need some coaxing with a little screw driver.
Just dive right in. It very simple once you get going.
Seriously. I'm here for BEER
It's Not The Size Of Your Rig That Counts....It's How Often You Use It.