Couple of things..
First, lets separate flexibility and mobility. What you want is mobility - the ability to move better, for your joints to go through a larger dynamic rom, and for your muscles to be balances and function well together.
Flexibility, on the other hand, is just one of the types of training that will help you increase your mobility.
First thing to do is to analyze for over active and under-active muscles, and ROM deficits. Once we've done that, we can next prescribe exercises to strengthen/activate the weak and lengthen the over active.
In addition, you can start adding in mobility drills and exercises that are more athletic specific. For simple starter mobility, I suggest checking out DeFranco's dynamic warm ups.
Br