if you plan on doing dressage for more than a year, invest wisely. The f.i.t.s. Are excellent breeches, though expensive.
Kerrits are a good middle level. They will last a fair long time. I do recommend full seat, and not the uber sticky kind. Just Clarino or leather. Those "sit tight" kind are TOO sticky. You will find it hard to reposition your seat.
I use Boinks. They are looser than other types, and less expensive, but do not last as long. They are much cooler for hot weather, so I like that.
Yoiu will probably want to buy half chaps, too. If you do not, you may find that the stirrup leather will pinch your calf as you post the trot. I love my half chaps. They add a bit of grip, and warmth in winter. Again, you get what you pay for. Ariat's are a good brand.