While mountain biking is a fun and exciting sport, it also has a lot of challenges. The challenges can come in many forms and unfortunately a successful outcome isn't always the end result. Mountain biking is a potentially dangerous and hazardous sport so you should understand and  accept that. Prevention is always better than dealing with consequences, but this doesn't always happen. We suggest you be realistic about that and carry a first-aid kit or enough first-aid supplies to handle an emergency if it happens. As well, carry a cell phone and have emergency numbers programmed into it before you leave. Tell a responsible person where you are riding, the route you are taking and when you will return.
The supplies you carry depend on where you are riding, your emergency first-aid knowledge and your willingness and ability to deal with the situation.
We are not medically trained people and therefore cannot and should not offer you direct advise. It is up to you to do some research and find your own solution on this topic. We have however found a website that has some good practical information that you may want to study. Most of the injuries they discuss, and related treatments, are "typical" mountain bike  injuries and so that will help you narrow your focus a bit. Be safe on the trails but when things go wrong make good decisions about the best course of action.

http://www.utahmountainbiking.com/firstaid/index.htm