At the first level my genuine suggestion would be to fill your squad at its maximum with whoever it is. You should have all 28 players. Then I would recommend focusing on increasing your training levels as your top priority.

Then keep accumulating resources from watching free daily videos.

Leagues in the first levels are boring, so you can just buy a few strong players (regardless of the age) and win with them. In several seasons time, when your training levels are high (ideally - 80lvl+ but I can't remember how long did it take me to reach these levels) you can start meaningfully build your team with fast trainers and enter a new 'level' of playing Top Eleven.

And keep an eye on your ground development progress.

Good luck