If you need something (or someone) blasted to bits, Carth is probably your man; his ability to dual-wield blaster pistols will make him a handy ally during the earliest portions of the game. This has less to do with any special aptitude for battle, however, than with the fact that he's your only ally for the first few hours of gameplay. After Mission and Zaalbar join your party, Soldier characters will want to kick him out to make room for that duo; Scoundrels will want to keep Carth for a bit longer, but after you rescue Bastila, Carth will probably take a permanent back seat for the rest of the game.
That isn't to say he's not a good character to have along in a fight; he deals decent damage with his pistols, since he gets two attacks per round, and his soldier vitality will help him last a bit longer against the tougher opponents you meet. It's just that he's good, not great, and there are other NPCs that perform the ranged attack function better than Carth does.
Attributes: Pump dexterity at every fourth level. If you don't plan on using Carth after Taris, or are saving levels and desperately need someone to engage your enemies for you, give him a point in strength and let him go crazy at point-blank range; just don't expect him to excel in this function.
Skills: Carth gets the same dismal skill progression that's common to all soldiers. The only thing you should ever need is treat injury; you'll be relying heavily on medpacs until you find Bastila.
Feats: Carth will get more feats than most of your other NPCs, luckily, so use these to maximize his effectiveness early in the game. He'll start out with decent feats to augment his dual blasters; what you should pick after those are feats that will give him a standard boost to his combat efficiency, like toughness. Active combat feats aren't a good idea, since most of them will reduce his attack, and he'll already be taking big penalties for dual-wielding weapons.