I'm going to give you "the talk".
First, sidearms are not completely necesarry unless you attend a venue that requires one in certain areas (bunkers, killhouses, indoors, close range, ect.). If the field you attend does not require one, or if you just play with your friends, I would recommend holding off and saving your money. For $100, you could buy a couple extra batteries and a lot of extra mags for your primary. I'm a firm believer that access to hardware that keeps you in the fight longer makes he experience more enjoyable. The key to winning in airsoft is a wall of bbs, not variety. Second, if you feel you REALLY need one, either go very cheap ($40 for a Crosman CO2 pistol) or get something worthwhile and expensive, like a Tokyo Marui. Me and my buddies have gone through cheaper gas blowbacks (WE, KWA/KSC) and I've never been satisfied with them.
Bottom line: if you are new, get more mags, ammo, and batteries for your primary.