For Intro level you need light colored breeches so tan, light gray, etc will work. Think of the closest/lightest color you can get to white. The jacket color should be Navy or Black, preferably not pinstriped though again, Intro tests aren't as stringent and if you are going to a schooling show pinstripes won't matter. You need to wear a safety helmet. The shirt should be white or light in color, short or long sleeved..not sleeveless. If jackets are forgiven due to heat but you are wearing a sleeveless shirt, you would still have to wear the jacket. Yu can wear either the standard choker type collar or can get a stock tie..preferably one permanantly pre-tied as it is much easier :)
As for the rest, boots, either field or dress, I am making the assumption of an adult rider (18+) as I have never seen a dressage rider in jodpurs and paddock boots though I suppose they must exist as ponies do compete. As for half chaps, only leather type is allowed; no suede.
At the lower levels you can use a hunt seat or all purpose type saddle so the color difference between a brown shade or black isn't an issue. Just try not to mix the colors between bridle and saddle..go all black or all brown...it just looks better in any case regardless of type of riding :).
Use a conservative saddle pad color..white or black preferably regardless of color of tack. It can be a square pad rather than a fitted hunt style pad.
In a schooling show you don't need to braid but in the case of a longer mane, you can try a running or french braid of the mane.
or do a search on a running braid...neither are extremely difficult but they DO take practice.
On your tests you are looking at Intro A and Intro B..these are your walk-trot. You can do a search on YouTube for watching someone ride the test..but here is a decent Intro B:
and Intro A:
You can find the tests themselves at USDF | Downloads | Forms and Documents
You are limited on the types of bits you can use in dressage so you need to make sure you are using a legal bit.
Here is the rulebook: http://www.usef.org/documents/ruleBook/2012/08-DR.pdf
Even if you are only competing at Intro level, this is a good reference guide if you want to learn more about dressage and why it is done.