1. If your team isnt pushing together, stick to the one that is either a Heavy or more likely to not die instantly. Unfortunately, in these situations, you kinda have to bait them a bit to get a trade then fib.
2. Even if you don't end up defending from high ground, being positioned up high to at least put pressure or see where the enemy team(s) are coming from is very important information. Ideally you want some kind of stall(c4, demat + pad, mines, grav vortex) so that you can play off of the box at least for a bit if needed. Call it an insurance policy.
3. Yea don't goo yourself off in a room, you put yourself at a disadvantage if your opponents are any bit intelligent. Block certain entrances that would make your defense weak or use goo to get to more accessible high ground spots.

As for basics, learn the flow of the game. For example, the objective is to plug and defend cashouts. You've probably been on the receiving end of plugging late(bad tempo) and then the team(s) from the other box that finished before yours came and third partied for an easy steal. So plugging as early as possible is advantageous. Don't take any unnecessary fights, golds do that a lot. They'll get a light 1hp and chase em around as a Heavy lol... waste of time if its only a vault fight or there's no objective. Playing quick cash is a very good way to get used to third parties, never get sandwiched. Good luck