You can wear maternity clothes as soon as you want to. In your first trimester (3 months) you can usually get away with wearing your regular clothes. You might need to get a bigger maternity bra though, as comfort is everything when your expecting. As you move into your second trimester you may need to consider buying youself some nice maternity clothes. Its always good to get some staple pieces for your wardrobe. A nice pair of pregnancy jeans or maternity leggings themed with a pretty top, possibly even a nursing/ feeder top. This will last you all through your pregnancy and beyond.
If you have an occasion a maternity dress is an essential buy. But you dont have to go mad spending. Mamalicious and jojomamanbebe have a lovely collection of maternity clothes and dresses to choose from, and all at a great price too.