At your level it's best to start with 4-4-2, especially without an AML, you don't have a lot of options.

4-4-2 is the most balanced tactic on the game so when you get to grips with it, you can try other formations too.

As for players, say you have three 3 star strikers, and you sign a 5 star striker. You can choose to sell a striker you don't need.
Take into consideration age and quality and sell the weakest striker. Just an example.