Fill a bucket with soapy water, and use a scrub brush to clean the brick -- removing any accumulated debris. Let it dry completely.
Vacuum the brick, to pick up any remaining dust particles stirred up from the scrubbing.
Pour a small amount of paint into the bucket, and add water to thin it out substantially. The paint should be very watery. The objective is to lighten the brick -- not cover it with paint. Because brick is very porous, it will absorb the thinned paint rather than create a seal.
Apply the diluted paint, using an old rag or a paintbrush. Using a rag will allow you more control over the amount of paint you're applying. It's easier to further lighten the brick later than it is to darken it.
Allow the brick to dry completely after the initial, light coat. If a second coat is required, apply more diluted paint to suit your personal preferences.