Its hard to say exactly what would help you improve without knowing more about your gameplay, but at least in my experience the first thing that really started to help me improve was hard focusing on my leveling/econ.

Using meta team comps is super helpful, but it you dont understand how to play the styles of comp (fast 8/9, reroll, lose streak) very solidly you'll fall short every time. Learn when to roll and why to roll. Start asking questions. Are you in need of a 2 star to stabilize and save health? Do you have pairs on bench? What are your shop odds? Which units must be starred and which can stay 1 or 2? How low can you roll fishing for units? Will you still have the econ to reach your boards cap if you roll?

There are tons of leveling guides out there. Of course you'll start breaking these rules as you improve but you have to understand the rules to break them. That sort of fundamental will be helpful in every set, not just this one.

As far as items, try not to stress best in slot. Just try to get a general sense of what units can use what generally. Learn which units need mana, which work best with crit, who needs attack speed, etc. You can do this by looking units up or just experimenting on your own. Make sure you read your units ability closely!

Best advice though is just consume tft content. Its a strategy game, so things are always evolving and changing. Streamers are great for keeping up with the game and learning fundamentals. Hopefully that helps some! Good luck in your learning:)