I can help a little.
I haven't tried a firearms character myself yet, but the way you build tech items is by raising your skill in the relevant tech college. In your case, this would be the gunsmithing college (as opposed to electrical, mechanical, etc.). For each skill level you earn in gunsmithing, you will learn a schematic, which will let you build a gun, or ammo, or something relevant, out of parts. There are other schematics in the game which you can find or (I think) buy. I know you can buy schematics if you are using the unofficial Arcanum patch but I'm not sure if that was in the base game. Many of these "found" schematics require a certain amount of skill in two different tech disciplines, like gunsmithing and electrical, for example.
Anyway, to actually build something from a schematic, there's a button on the UI called "schematics" (it looks like a gear) that will take you to all the schematics you have. If you started the game with some gunsmithing skill, you'll already have a schematic or two. From that screen, if you have the right ingredients and enough skill, you can make the item. Some followers can make things too; to have them build something for you, I think you just talk to them and one of the conversation options involves asking them to make something.
In terms of building your character, you will definitely want Intelligence, as you need high Intelligence to be able to gain higher skill levels in gunsmithing. To shoot people, you will want firearms skill, and a half-decent Perception stat. Other stats that are useful are Dexterity, which gives you more actions in combat, and Strength, to carry more stuff. But you could also recruit followers to carry all your stuff. You may also want to think about other technological disciplines you could choose along with gunsmithing. You will definitely have enough character points to try two tech paths, at least.
It's goo to know that the early guns in the game are pretty weak (think Fallout) so you will want a backup weapon / followers who can fight for you. Later on guns get better, but I've heard they're never really as powerful as one might think. General consensus seems to be that grenades are much more effective. I've tried a grenadier and had a pretty easy time.
Finally, remember that being a tech character means that magical things won't work well for you. This includes healing magic. You'll want to use tech healing salves rather than magical healing potions, and NPC followers who try to heal you with magic will have a hard time. Also, enchanted items will basically have their enchantment not work for you at all. On the plus side, you can use technological items, like machined armor, better-crafted melee weapons, and, of course, guns, really well.
General consensus is that magical characters are easier. I've only done a melee character and grenadier so far, and they weren't too hard. I found that a straight-up melee fighter gets boring though. The game can be tough in the beginning, but I recommend sticking with it because it's really rewarding later on. If you find that guns are giving you a hard time, maybe restart with a different build until you get a better feel for the game. I get the sense that a gunman might be easier on a second playthrough than on the first.
I hope that helps.