How does a character gain his starting skills, I cant seem to find where this is explained. My Guardsman has Drive (Ground Vehicles) and Speak(Low Gothic) and I didnt spend any of my 400 starting xp to learn these talents. Am I doing this right?

Think of it as basic training. It's just something everyone of the same career path receives. The Speak Language skill is obviously something learned quite some time before that though.

Yeah your starting skills/talents/gear are just something you have for being that class, it doesnt cost any XP.

the 400 XP are for customizing your character after taking the initial training.

Speaking of starting skills, does anybody else think it's silly that Techpriests doesnt automatically gain or require Common Lore Tech and Mechanicum? You get them as basic skills if you're from a forge world, but I still don't think that's good enough as a tech priest. I offered them for half the price to my forge world techpriest.