I currently event at Novice with my mare, but, you would need to get an experienced eventing trainer. Next, make sure that this is what you want to do because eventing is VERY money consuming and it is very draining. You will need to ride as much as you can. Also, make sure your horse is fit and sound for eventing because not all horses are sound for eventing as it is a very high-intensity sport
First, you should get a trainer/instructor. They will focus on anything that needs to be improved or corrected and help you become a better rider. Do a lot of cross country schooling to get used to terrain, water, different kind of jumps. Start with lessons, then clinics, then unrecognized events, then recognized events and so on.
Start off by getting yourself a trainer or an instructor- they will know how to help you.
With young horses I have always introduced them first to dressage competitions, then showjumping competitions as it will get you used to jumping under timed conditions.
Then I would incorporate some hunter trials as it will calm your nerves on a XC corse which can often be very overwhelming for people.
Then I would often enter fir a local ODE hosted by a riding club or pony club and away you go.
If you simply want to event for fun I would continue going unaffiliated. But if you want to (a) go higher level or (b) want to qualify for an event or be in a league I would go affiliated. Moreover if you are producing a horse to sell, it’s better to have an affiliated record as it seems more professional, hope this helps x