Two ways to do this: with a pre-emergent, or "weed preventer", which needs to be applied now; and with a post-emergent weed killer for broadleaf weeds. A severe weed problem will require both. Garden centers should carry both of these products. You apply the weed preventer with a spreader, and the post-emergent weed killer can be applied either as a spray or as a granular with a spreader. The spray works better, but is more work. The stores in your area should carry the formulas designer for Southern grasses, but check the label to be sure. Some weed killers can damage St. Augustine, so you need be sure you have the right stuff. And be sure you follow all label instructions.