I'm still pretty new also, so take my advice/opion for what it's worth.......
You'll for sure want to train 3 players all at the same position. You'll need to decide if you want to train guards of inside players.
Right now, I'd advise you to work on building up your bank and saving as much money as possible instead of going out and buying trainess off the wire. With only about one week left in the season, there's not much on the wire, and if you buy an 18 year old he will be 19 in a couple weeks, and a 19 year old will be 20, so it's really like a lost season or two of training. You'll be better off trying to get some good players from the draft, though I'm not sure you have enough time to build up scouting points to find the good players in the draft.
In a few weeks from now, right after the S19 draft has taken place, the wire will be stuffed with 18 and 19 year old trainees and you'll be able to buy them off the wire if you don't get what you need from the draft. If you want to tell us what your current 18 year olds look like, I'm sure you can get a lot of help here on if they're worth training and in what direction you will want to train them.
I will say this as a fact, this game is very complex and training is very complex, make sure to read the info available in the Help section as well as what you can find on these boards before you make any decisions. But training is, in my opinion, the most important part of BB. Sure, you can buy already-trained players on the wire, but training your own players has advantages, not to mention cheaper.