Even though your lawn has filled with weeds it is not too late to apply a pre-emergent. The weeds you have now will put on a flower and drop seeds to sprout more weeds. A pre-emergent will not kill the existing weeds but will prevent more from sprouting. Broadleaf Weed Control with Gallery is recommended for most broadleaf weeds and Weed and Grass Stopper with Dimension will kill some broadleaf as well as grassy weeds like sticker burrs.
Once our weather warms up to 70-80 degrees with no rain for 5 days or more it will be time to apply Weed-B-Gon for Southern Lawns to kill the existing weeds. It is safe for Floratam St. Augustine and Bermuda. Give it 2 weeks to fully work and then it should be time to fertilize the lawn as well. Remember that fertilizer is taken up with active growth, so until we are mowing consistently every week for 2-3 weeks it is too early to get the full benefit of lawn food. Be patient with your spring lawn care. We all want to see results and have a pretty lawn again this spring.