Swimming is required for almost every event - swimming pace is much less important than being able to swim while managing a full backpack. Adventure racing always teaches you new skills. Navigation is probably the most critical. To work on this, contact your local orienteering club. River reading skills are another key element.
Instruction is generally available wherever commercial rafting trips are offered.
Rope skills required for racing are generally rudimentary - check with climbing schools in your area for instructions. Practice these skills thoroughly. Equipment selection is dictated by the race - the terrain and climate, the expected disciplines and the safety requirements. Heed the race organizers' suggestions - if some element is required, there's generally a very good reason why. Within the guidelines and requirements, you obviously want to minimize the weight you will be carrying.