For a revolver, stick with 38 Special/357 Magnum. For a semiauto, the 9mm is a great choice, never a mistake.
Avoid oddball calibers, it just means paying more for harder-to-find ammo. For a newbie, stick to 38 Special, 9mm Luger, 45 ACP, or 22lr for your first handguns. If you only buy one, the 38 Special or 9mm are both fine choices.
Don't forget to look at barrel length, sights, etc, and plan on putting good coin into a belt and holster. A hundred bucks for carry leather is not excessive.
Do get 500 rounds of ammo, plan on shooting half by end of 2015, the rest in 2016.