Don't buy big fish as babies and put them in a small tank, and say you're going to buy a bigger tank when it's time. It usually doesn't happen. Unforeseen circumstances always come up. Money troubles, or space issues, etc, but usually you end up with a cramped fish in a small tank, except now you've gotten attached to the fish so you don't want to let it go to someone else, or it's too big so no one even wants to take it off your hands in the first place. Save yourself a whole world of hurt and just get the final tank first.
Don't leave the heater above water (sometimes it happens during water changes). It could explode. It's a long, traumatic story... I don't wanna talk about it.
CYCLE! I know you might be anxious to get your new fish home, but it's just gonna end up dead if the tank isn't ready. I killed so many fancy goldfish as a beginner just because I didn't know anything about cycling.
Plecos are not good cleaning fish. They're a fish, just like any other, and so they require just as much diversity in their food as any other fish. And they poop everywhere. You know those annoying spray cans that shoot out strings of foam all over the place? That's what a pleco is to the fish world.
Keep up with water changes. Even if you're exhausted... well, you wanted fish, right?