what is the name of this small plant with yellow flower?
This looks like Sphagneticola trilobata also known as Singapore daisy, creeping-oxeye, trailing daisy, and wedelia.  It is now found throughout the Neotropics but is native to Mexico, Central America, and the Carribean.  It is a weed that is classified in the top 100 worst invasive species in the world.  



Source: Sphagneticola trilobata (ePlants)