I can't give any proper advice without seeing the injured digit, but I can give some generic tips as a man accustomed to minor injury (daily fare of someone who does martial arts I guess).
Is there much or any bruising? If not chances are it's just a bad sprain which will heal quickly with rest and ibuprophen. It might also be worth putting some ice on it today to try and take down any swelling. If it's severelly bruised then it could be broken, in which case treat it as above, but also get yourself down to A&E for an x-ray. They may also strap it up for you, which would stop you playing guitar for a while but would increase healing time. Which has to be worth it in my book.
Hope you recover soon dude
*Rahnooo*