Well it is true that both training youngsters and expanding your arena are good ways to start, but I think that it is much better to expand your arena at first(which pool clearly shows, it is 11-24 atm for arena expanding) especially with this new changes about overextension tax... Trainers are now much more expensive(and if you are not following that market, prices are going crazy atm:D), buying good trainee is not cheap either, you have to put a lot of hard work into his training, and in the end if you try to sell him, you probably wont get any profit, maybe even lose some money with him... Only way to actually earn some money is not to sell him, but to continue to train him, and get overextension for him, but salary for fully trained 22-23 year old is probably too much for low league club, which didnt expand his arena, is too big, and you will be forced to sell him...
To conclude, I believe that much better way to start is to expand your arena, and I am pretty much a newbie here, so I know all the problems newbies has to face:)