Re: when can i compeat?

Some riding schools do, but some don't. Some may allow you if it is Pony Club and they are a Pony Club centre, and some may if the owner or instructor is there. This is something that you'd have to work out by yourself, maybe ask your instructor or the horse owners? Maybe you could borrow a friend's horse if they would let you?

As for competiting, I believe that you can compete at local shows whenever you are confident. My tips would be to go on a horse that you have ridden several times before and you are confident riding this horse. Also, choose to do a dressage test that is suitable for your level, but not overally challenging, so that you build up confidence. Make sure you know the rules of warming up, such as passing left to left, and the meaning of different coloured ribbons in horses' tails. If possible, go to some other local shows to get an idea of what you would be doing and make sure you feel comfortable with this.
If you compete, good luck!