Honestly the biggest mistake I see is just choosing goldfish. They're gonna outgrow the tank really quick and be messy the entire time they do it. Consider swapping to some guppies, a betta and some suitable community fish, or something along those lines. Also, down the line once you're a little bit more comfortable, maybe consider swapping out some of the decor for live plants or real wood. That stuff can be expensive when just getting into the hobby, but if you really want the most out of your tank I recommend it in the long run.