The four most helpful pieces of advice I have are unfortunately all obvious:
1.
Play a lot of games against players who are at least slightly above your level.
2.
Understand the core tactics used by players; one way to do this is to look at the games played on those ladders by top players/teams.
3.
Read a strategy guide or two -
https://nghood.com/warlight/strategyguide is the gold standard.
4.
Revisit and analyze your games, especially watching for surprising or decisive moves.
Good luck. The team ladders do have a reputation for being easier than the 1v1/Seasonal/Multi-Day ladders due to low participation, so it might be a gentler learning curve.
Edited 3/2/2021 17:35:58