Newbie, first decide what size gun you want to carry, what caliber, how many rounds, doa or sa trigger, safety or no safety. Find a gun which fits your hand. If there's a range near you that rent guns, rent a few to get the feel. Don't be to cheap, there are many good guns in your price range. Ruger, SigSauer, S&W, and many others have decent guns in the $450 range. Like you, I have a Smith-Wesson Sigma 9mm, however, I usually carry a p250 by SigSauer in 9mm - 15 plus 1 rounds, or a Taurus p709 also in 9mm - 7 plus 1 rounds. One is a compact and the other is a sub-compact. Once in a while I'll carry a SigSauer p226 which is full sized. Which gun gets carried depends on my clothing for that day, Full sized usually with a jacket of over sized shirt, compact with regular shirt, and sub compact with a t shirt or other summer wear.
So check out your different options, you'll find a gun that suits your needs. Just make sure, what ever gun you choose PRACTICE, PRACTICE, PRACTICE, and ALWAYS CARRY.