In Australia there are many pony club one day events if you are a member of pony club, and also "chicken days", I am sure there is something similar where you are from.
Basically you just turn up and there are a few low levels that you can do, like only 60cm (2 feet) jumps and prelim dressage. Sometimes there are even lower ones. Generally they are pretty casual and people don't go all out in getting prepared, the cross country course is short and the jumps are quite low and achievable so your horse doesn't have to be mega fit. You should try to find some of these to try out, just to see if you like it.
At home there are things you can do to prepare. Dressage is the basis to almost all english riding, and in top levels your success is dependent on your results in the dressage. Its the first event and you can lose heaps of points. So if I were you I would practice lots of basic flatwork and dressage, and just getting your horse soft and responsive.
Then, if I were you, I would go out to open fields, and work on some large circles and even straight lines, ensuring your horse was responsive and calm as they were in a flatwork area. I had OTTB and he was fine in the arena but we would go out to a cross country course and he just would try and run and run. We did some low events but it was always a fight. If you can get your horse responsive and calm on a cross country course you are going to be a step ahead.
Then practice your jumping, build some solid fences, like little hedge things, barrels, tyres, and practice doing some drop jumps. If you've done cross country before you should be fine though.
I don't think you really need an eventing instructor. If you don't feel confident in either your dressage or jumping ability, then just get a dressage or jumping teacher, and they can help you with that. Cross country is basically show jumping but faster and over solider things, and if you have experience cantering across open hills and such it shouldn't really be difficult at all.
Start low though and work your way up.