If you haven't tried MTB Orienteering before, take a look at some of these useful resources:
If you're an occasional rider note that we have an expectation to carry a cellphone in non-urban events; this is to hasten response to an injury incident. And you are now allowed to ride off-track on certain map colours — read the legend carefully!
Next, read the Riding Hints, and then some Expert Navigation Advice.
See also About MTBO.