Don't worry about screwing it up, you'll be fine. I had barely touched an AR before I built mine, and I haven't had any problems that you could really call a problem. There are step by step instructions out there that make assembling one insanely easy. You'll still have anxiety though. Anxiety due to not being able to wait until it's done.Ive been looking them up for months now, considering doing a build or just starting off with a plain Jane AR to get familiar with. By my math, the savings on an AR build plus the anxiety of screwing it up and being overwhelmed by all the moving parts just isnt worth it to me. I personally think I need to feel the gun, explore it in its assembled state and learn how to break it down for cleaning and personal knowledge of the gun.
I only had to order one tool to assemble mine (stock wench aka castle nut wrench), the rest are pretty common and you may already have them. The hardest part about building one is deciding what parts you want, finding them, and staying within budget. I am convinced that I will never buy a complete AR now because building one is much funner.
I hear the upper can be a little harder to assemble, but if you buy a complete upper, you won't have to worry about that.
If you are set on buying a complete one, I recommend PSA.