All those mentioned here are good options. All I can suggest is if you do buy new, buy from a dealer. A good dealer is worth his weight in gold especially if you don't work on your own o p e. If you do buy used look at what brand dealers are close by in your area and try and stick to those brands as parts availability is important.
Everyone here will tell you there is no true one saw plan. Depending on how long you plan on cutting a big saw will get heavy if you do a lot of limbing and a little saw will leave you wanting when bucking rounds. I say get you a good 55-65cc saw and that will do you if all your doing is cutting up a single tree up to 24". If you are clearing fencerow or hedges then a 40-50 cc saw would be ideal because of the lighter weight.
Being your first saw purchase, I highly recommend buy new, and get some safety gear, especially if you are uncomfortable or scared while using it.