What’s the best way to get a used car in Houston?

We're new to Houston and to the US. O saw several ads on TV for names like Auto Traders, Auto Nation, etc. These appear to be large scale and reputed dealers. Also, doing a Google search shows many different dealers who I suppose are not too massive like the earlier ones. What is the best approach to go about this?

I'm looking for a used and certified vehicle with a finance option.