Intro? Are you in the US or a different country?
Here are some US eventing links-
www.useventing.com
http://useventing.com/resources.php?section=linkshttp://useventing.com/start.php?id=26It has three phases, as you know, dressage, xc and stadium.
there are six levels of "horse trials". three day eventing doesn't begin until the Preliminary level
BN-beginner novice-
w/t/c test,
2'7" fences,
350 meters/minute pace on xc,
Small up bank or a small ditch/run through water on XC
Stadium is basic, one certified combination
N-Novice
w/t/c test, slightly more complex
2'11" fences,
350 m/m
Unbank/down bank, 1-2 xc combinations, run through or upbank involved with water
Stadium has 1-2 combinations
T-Training
W/T/C, changes within gaits, 15m figures, stretch circles
3'3" fences
420-450 m/m
Bank cominations, water combinations, jump combinations, more challenging fences, half coffins
Stadium has 1-3 combinations including a triple
P-Preliminary
Changes within the gaits, back-up, Canter to walk, 10m circles, counter canter
3'7" Fences
520 m/m
More complex combinations, jumping into water, corners, large trakheners, coffins, Wide fences prevalent, introducing skinnies
Stadium has 1-3 combinations with a triple
Thats the first four levels, the others are Intermediate and Advanced...Advanced with 4'3", which doesn't seem like much, but try saying that when you walk the courses!
There are 10 Areas, Area1 starting in New England. They each havea website, and through that, barn websites and local tack shops, you can find a reputable eventing trainer. Often its best to train with a dressage trainer if jumping is your strength.
Make sure you have access to fields to conditon in, and hopefully a good sized hill too

Events at the lower levels can cover one to three days, depending on the size. One days are best to start at, so that way you don't have to worry about stabling, etc. Enty fees range from $100-200, and the scene tends to be very relaxed and friendly.
any questions, just ask. good luck!