First, I'd evaluate the current squad. Who isn't in your plans? What kind of football do you want to play? Who is the star of the team?
If a player isn't in your plans, try to move them on, loan them out, etc. Get them off your wage budget to free up money for prospects! After you see who is good and who isn't up to par, try to sort out a tactic that will fit the star players and that matches what kind of football you want to play. Remember, its probably not ideal to have a slow paced team playing fast paced counter attacking soccer. I'd attempt to build a tactic around your strongest players.
Next, for transfers, I'd recommend using the Team Depth Chart to decide where the squad needs strengthening. Based on your formation and the players that fit into the roles you have in the tactic and match the style of play, identify the weakest links and attempt to bring in some first team players to fill those spots for immediate effect during the first season. Once you get a solid foundation of a team and feel comfortable, only then I'd suggest bringing in the youngsters for future success.
The best way to develop youngsters is First Team Football. For your brightest prospects, I'd have your best players tutor them and sit them in your first team squad (assuming it already isn't a bloated squad). Whenever you have a cup game, think about giving the player a run with the first team. If you are doing well in the league and are playing against a weaker team, give the young prospect a game. If you're dominating and he is on the bench, bring him in as a sub. He will develop fastest with first team football. Tutoring helps with personality, player preferred moves, and determination stat change. If you don't see room for youngsters in your squad, loan them to a team where you know they'll be a first team regular or higher and played in the position you plan on playing him. The higher the level of football, the better.
I'm sure some people will be able to suggest more specific ideas than me, but I think every save turns out differently, so everyone's situation is different so its best to evaluate and decide yourself! Goodluck in your save and if you'd like more help/ideas, I'd love to help!