Quick life lesson I learned....popular guns are popular because they are good guns for most people. So, a gun like the Sig, S&W 22 revolver, a Glock 19.....should all be considered for that reason alone.
Calibers: Well, for a first gun, self defense shouldn't be a primary driver. Cost of ammo is always important as is general power level. For these reasons, I like 22lr and 9mm. Both are comparatively cheap. 22 has the added advantage of being able to shoot a lot before becoming recoil sensitive....like forever. 9mm is moving up to working cartridge power level. It will give you enough recoil that you will have to train to handle it where 22 LR does not.
Barrel length: Generally barrel length relates to sight radius. Sight radius is critically related to how far you can hit things. I can hit the target with a 10.5" barreled revolver with relative ease. With the same gun in 5.5", it is a challenge to make hits at all at 75 yards. 4" revolvers and 5" autos are great to 25 yards.
Semi vs revolver: This is a challenge. I belive revolvers are simpler to use, but many would argue with that. I can say I have owned many handguns; ~50% of the semi's had a feed issues, none of the revolvers did. Also, S&W's have easy triggers to shoot DA or SA. That cannot be said about many semi's, especially the DA/SA type.
Last, something to remember about a first gun is it is not a do-it-all, one-n-only gun. It is a learning step to get you from newb to experienced with one gun. Most guns can do this well and the only possible real negative is getting a gun which goes in for warranty work several times or has a fundamental flaw like a grip too big for you to hold and pull the trigger efficiently.