one thing you'll run into is tactics. Just so you know, 90% of BB runs Look Inside and Man to Man. It's easy to build a team around this because there are many players for sale that are designed for a Look Inside offense (e.g. guards with low scoring ability but high defense and passing, bigs that have high IS but absolutely no Shot Blocking, etc.)
So training offensive (high JS) guards are useless ?
Or high defense and passing guards are oversupplied because of that ? (So cheap when selling)
No, not at all. Defense is of course desirable but guards that can score as well are just as fine at lower levels. I can't speak to the situation in your specific country or league, but the talk of the dominance of inside tactics is something that becomes more important as you advance - at lower levels, often just being able to field a solid defense, manage enthusiasm and manage game shape is enough to be successful.
Training five guys instead of just two or three can work as well - it's much more important to learn how the training system works, get a feel for how you want to play the game and make sure that you're not spending a lot of money early on unless it's being spent on upgrading your arena.
The main pieces of advice that you should hope to hear are:
1. Invest in your arena, not in expensive players (either TL price or salary, especially not both!)
2. when adding players for non-training positions, look for guys with secondary skills at lower salary
3. invest in your arena!