We are a family of eventers. Make sure that your horse is use to field obsticles, not just jumps and ditches, etc. but people and other horses running around and even loose. Make sure they are use to you being nervious and tight in the saddle, the first time out you will be a tight fisted scarred cookie. Make sure that you can jump 2'6" easily. There is very few CC events that have intro jumps. Look around for a non-recognized event that has an intro level. When you are ready go for it. There is no other feeling like you and your horse taking care of each other on the field and finishing it in one piece.
You honestly don't need to be worrying about water jumps if you're first starting out. They have different levels of eventing. I went to my first combination test this past spring and it was so easy, I probably would of fell asleep if it wasn't for the fact my horse was pulling my arms off cause he wanted to run. I would try local schooling shows, doing GAG(green as grass) or beg. Novice if you're up for a challenge. On your first test, don't worry about making time, just enjoy the ride and let you and your horse get a feel for it. Your horse should be jumping about 2'6", like stated... always practice higher jumps than what you're going to be doing at shows.
As for attire, nothing fancy. A safety vest is a MUST! They won't even let you compete without one. As for show attire, just a nice polo, boots and chaps(or tall boots, doesn't matter for schooling) and of course, breeches... any colour. People have worn purple ones before at the schooling show to match their saddle pad/polo wraps xD
Just remember to have fun, you don't need to be too serious until you get higher up in competition.