Personal anecdote time! (Warning: long. Also mostly rambling.)
Yeah, so my first game as DM was during the tail end of the D&D5 playtest. And it was bad. Well, my friend had fun, but I think the others were just counting down until it was over.
The reasons for this, I feel, are twofold: one, I was basically playing the thing by ear. I had a basic outline of where I wanted to go, but I was spending half of each week planning scenario and encounters and worrying about making it fun for everyone and trying to give my friend what he wanted while also rewarding everyone else...
Oh, and the DMPC (who had been my PC in the previous game, and was thus sufficiently minmaxed to take on the entire adventure solo) probably didn't help matters.
The other reason was that I really had no idea how to build an adventuring day. I swear I seemed to think that if the characters weren't half-dead from the first encounter of the day, the encounters clearly weren't hard enough.
All of this could be avoided if you just use a pre-made adventure, of course.
My second adventure was much better: planned rigidly from the beginning, no real side-paths (just a single dungeon), better encounter pacing, only three good friends playing, a comprehensive set of rules to go with (as 5E had actually come out). Basically, it fixed everything that made my first try such a catastrophe. Plus I think the players were excited to be using 20th-level characters (yeah, don't try that. 20th-level characters are very difficult to constrain).
Since you're playing Pathfinder, you have a fair bit of system bloat to deal with - prestige classes got pretty ridiculous in D&D3.5, and that carried over to Pathfinder. You may want to limit your players to just the core classes until you find your footing.
tl;dr it's not too hard, as long as you accept that you'll probably mess it up.

I'll take any advice I can get! Thank you very much!

I've played a couple of D&D campaigns (AD&D, 3.5e and 5e, specifically) and it seems to me that there are two parts to character creation: fluff and mechanics. Fluff depends on your creativity, mostly, so the time it takes changes from person to person. Crunch (mechanics), meanwhile, mainly depends on how familiar you are with the rule system.

I've only JUST started learning, but I think it will be quite fun!

Will do.

That's a beautiful piece of advice that I will write and hang on my GM screen.

I love finding reasons for things and developing back stories; that will be quite fun!

No problem!

Thank you very much! Do you happen to know a site that has dungeon maps and good beginner's scenarios? Until I get my feet under me and learn the rules, I'll be using mostly pre-made stuff, although I might tweak the story as I wish.