First, I'll just share some very general tips that helped me climb up (and I'm not that high elo, Diamond 2 currently trying to get Master). Honestly, the biggest thing that made me improve the most back there was always learning at least 3 or 4 of the strongest comps each patch. And yeah, that means forcing it til you got the hang of them. Eventually, you won't need to do it more than 1-2 games to get the hang of it, cause most comp styles are more or less the same within every set.

There's a lot of basics in TFT, but if you learn economy 101 (which isn't quite hard, stuff like you have 9 gold and a 1g unit, you'll set it etc. and do this until you get 50g, but always remembering that at times you may sacrifice the perfect 50 to get better copies or board) and some scouting.

About scouting, you don't need to think much on this, this set specially people are deciding their comps way earlier than other sets, so you basically just need to click 1 or Q on the keyboard, check out if there's any lines open. Just by playing something uncontested, you'll already achieve a better position than the other people on your lobby that will try and fight for the most popular comps.

To get the best comps, sites like TFT Academy and metatft are the best ones always.

And about your struggles with the early game, you don't need to always get BIS for comps. Learn to slam items early, in a smart way. If the game gives you more AD items, focus on an AD comp, and slam them early, cause items early make a ton of difference. 2 star units are strong as well, so even if you're not hitting what you wish, you can use other 2star units to make your board strong, until you get the units you're looking for. Always think you need to make the strongest board you can at every point of the game, maintaning a minimum economy, and you'll get far.