There's plenty of guides you can read on here, have a look at the stickies on the Tutorials thread.
But if you'd like some advice from me, I'd say that rather than concentrating on one formation, and buying players to fulfil that formation only, it might help to buy players with multiple roles, and add some depth so you can play different formations.
So you're going to want 2 GKs and at least 4 DCs, 4 DR/DLS (it's best to buy multirole defenders like DC/DR)
A couple of DMCs (Again, DMC/DC or DMC/MC works well, if you don't usually play DMC, therefore you're not buying players you'll rarely use)
4/5 MCs, and I'd say 2 MR/AMR, ML/AMLs each.
Get a couple of AMCs too (again, multirole works well, eg. AMC/AMR), and at least 3 STs.
In other words, you want a squad that can play all positions, so you can play a wide variety of formations.
Just a note on the 3* players you're buying - they'll go down to 2* when you get promoted, and I'm guessing you're wanting a bare minimum of 3* players in your squad, so it'd be wise to invest in 4* players now.
You can actually pick up some really good 5* players at the end of the season, when the transfer market calms down.
Bit long winded, but like I said the in depth guides written by some other forum users will help you much more than I can.
http://forum.topeleven.com/tutorials...ven-guide.html
http://forum.topeleven.com/tutorials...-part-1-a.html
The second link is a good team builder tutorial.