For the NT, it's a very long term plan (10 seasons, ~2 real years), it's a huge commitment so make sure you start with a very good trainee. His potential needs to be MVP or HOF.
A slightly shorter plan would be to train players for the U21, it's only 3 seasons of training so it's much more manageable when starting, and a player with great starting skills (50-55 total skill points at 18) and an average potential (perenial allstar or superstar) is cheap and will give you a nice paycheck if you sell him at 21-22yo.
But like other said, priority #1 is expanding your arena