I should buy an impact driver! I use a drill and I have to predrill all my sheet metal screws.I'm a beginner, so please listen to the other people here for serious advise. But I wanted to say, I had a drill and I was really struggling. All the self tapping screws were getting stripped heads and I wasn't breaking through the metal. I figured there had to be a better tool, so I found an impact driver, Dewalt, for $59 @ lowes. It's just 12V, and I guess I wouldn't have bought it had I realized that wasn't optimum, but really it's works great. All the screws go in fine, and I've not stripped a single screw head. It's small so I can get in odd places. And It makes a great sound. I just have the battery & charger it came with but it's lasted all day every day so far. Another tool that's been a godsend is a circular saw (skil from lowes for $39) and I got a kreg rip guide (Lowes, @ $39, and a triangle thing, I forget what that's called, but it was like $7 and it makes for perfect straight cuts. I would go nuts if cut lines intended to be straight weren't straight, a personal quirk. Those are my favorite tools that I use the most. I also use a jigsaw for curved lines.
For straight lines I always just clamp whatever straight thing I have handy to use as a guide. I bought squares and never use them because they aren't square enough. I find measuring both ends of the cut produces perfectly square cuts. Is that kreg thing actually square to within a 16th over a few feet? Nevermind I just looked it up, it's not a square so much as a guide. Looks pretty cool though.