Firstly, ice is your best friend. The old mantra of R.I.C.E. still works a treat for acute injury - so bandage it firmly after using some ice wrapped in a wet tea-towel, and stick it above the level of your heart as often as you can in the first 48-72 hours.
Secondly, go and get it properly assessed. This might be your physio, your GP (if they have half a clue about sporting injuries, which *most* aren't too good on), or at a pinch the sports therapist / strapper (if you have one associated with your football club). This will give you the best idea of *what* it is, how bad it is, how long it will take to resolve, and what you should do to maximise your recovery towards playing sport again.