It's hard to tell from those photos but they look like winter weeds, and they may go away by Spring, but you can use a granule treatment such as 'Weed And Feed' or "Preen Weed Preventer" to get rid of the broadleaved plants like those in the pictures.

The larger one (pictured at the top) is dollarweed...yes, goes crazy! The smaller, more delicate-leafed variety (second photo) is all over our yard (Houston area, so, welcome to TX!) this time of year, too. I don't know what it's called, but I tend to pull these out by hand. I try to stay away from herbicides unless absolutely necessary, and these smaller weeds are easy to pull up. Stop by your local Home Depot/Lowes -- there is an herbicide on the market that will kill dollarweed but NOT your St. Augustine grass. Good luck!

thanks!

I pulled a lot of the second one by hand but they come back fast!

Yeah...the 'baby' ones are actually lurking all over your lawn right now, beneath the turf level. Just be sure to pull them before they go to seed.

The 'baby' weed in the second photo may be 'bedstraw', also known at 'catchweed'. It has sticky leaves that cling to everything making it messy to get off your clothes, dog coats, etc. when it gets larger. Bedstraw has a very slim connection from its wimpy roots to the stems making it seem easy to pull since it breaks off at the thin part. I'm not positive it comes back from the roots when broken off, although most plants do, but when the ground is damp the root will also come out if it's grabbed low.