Firstly you have to think how the injury occurred, so think if you can remember knocking it or hurting it, so think careful, even the smallest of knocks could have been the catalyst.
If not, then we have to assume its a strain on the joint, so ice to reduce the swelling, put the ice in a cloth and apply for a few minutes, remove, re-apply, remove. take care not to over ice the finger. Put on a small splint to immobilize it, this is simple, make a small tube out of some cardboard, tape or bandage it up around the finger. Make it so it is snug, but not to tight as you need to be able to slip it off and on when required, as in washing or icing up. It is to support the finger but stop the joint from bending. Then take some Ibuprofen to reduce the swelling. If possible keep the hand elevated above your heart, so touch your shoulder So it is ice, imobilise, and rest.
In first aid there is a treatment called R.I.C.E. which is the acronyms to remember if you get a sports injury... RICE, which stands for Rest, Ice, Compression and Elevation. Using these immediate first aid measures is believed to relieve pain, limit swelling and protect the injured soft tissue. You are using this as the split will act as the compression part.
This should feel better in a few days so stop the Ibuprofen after a day or two and switch to rubbing in an Arnica cream. use the splint to help rest it and all should be well in a couple of weeks. If there is no improvement go and see a medical professional.
On this forum there is plenty advice about stretches and warm ups, both are advised to do before and after playing so have a search.
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