does my horse have to know all this to compete

Sooo,Gidget isn't really what you call dressage material. She is a short,broad,stocky paint...not no long legged,elegant,lean horse. So I know that people might find it funny if I were to compete in small shows with her later down the road once I feel I am ready. But I was wondering if she needs to know how to piaffe and do that side trot thing and all those fancy moves..does she need a trainer for that? I want to be really good and I want Gidget to be really good too.She's a smart horse..well I think so...bit stubborn but not dumb.

If you expect her to compete in dressage, even at the lower levels, then she'll need to know all the correct moves and patterns.

So yes, you'll need a trainer to teach both you and your horse.

Thanks!

No. At the lowest levels, dressage is simply walking, troting and cantering normally in a pattern, called a test. You can purchase a test book from the USDF for a nominal fee, or you can download them off the internet on some sites.
It would still be very useful for you to have a coach.

You will need to get a trainer to do dressage, however at the lower levels it is within the ability of most horses.

Basicly if you can get your horse to walk trot and canter in a vague outline and do a circle then you can start with dressage. In the UK we even have affiliated walk and trot tests, no canter required.
The lowest level of proper dressage in the UK is prelim and for that you are required to show, walk, trot and canter, 20m circles in trot and canter. Free walk on a long rein, and I think a couple of half 10m circles in some of the harder tests.

Leg yielding, travers are required at elementary (the order of difficulty goes, prelim, novice, elem)
You don't require half pass untill medium level, with halfpass zigzags required at advanced medium.
Piaffe is not required untill PSG!

First, I agree - you need a trainer if you want to teach the horse (unless you are a very good dressage rider and know how to progress) and show and win.

Second, being short (if you are talking about the body, not height) in fact is a positive thing. My qh is long and it's a huge challenge to keep that impulsion. And let me tell you when she rounds and come on bit she looks very nice. I truly think any horse moving round and freely looks gorgeous (look at before and after pics some people (like say Kayty) posted - horse doesn't look nearly the same!).

Thank you everyone.

I do have a dressage trainer :)
I have a lesson tomorrow.

At the lower levels like they have at our county fair, you only need to walk, trot, canter, extend the gaits, change diagonals, and maybe change leads once or twice. Once you get into higher levels you will need to know Piaffe and Passage and Half-Pass, etc.