Sims 3 was my first sims game, and I'm not a fan of tutorials, so I jumped in and blundered my way through a lot.
I think the worst mistake was in my first game, when I didn't realize you could use edit town to move in other families. I thought you had to start with a family, and set them up, then switch households to add someone. I also didn't know you could turn story progression off... so I'd get really upset when I'd switch houses to set someone up and then go back to someone I set up before, and find out they got into a relationship or ruined a relationship with someone in their house.
I also didn't realize you could custom-color outfits for a while, or that you could change hair in the mirror. I know about the dresser, but ugh, my sims had some hideous hairstyles for a while. I also didn't know toddlers could use the dresser at all, so I usually cringed when I saw how my babies aged up.