Following these tips will help:
First, take off any removable parts like elastic bands and headgear.
Place your brush at an angle of about 45 degrees to the gum.
Use the same brushing motion on the inner and outer surface of all teeth.
Don't forget to brush the chewing surfaces of your teeth - and make sure you reach the back teeth too.
Finish by cleaning around your braces, brushing first from the top and then from the bottom.