Derivatives and tangent lines

I'm having some trouble on these questions. On other questions for this I was using derivatives to get the answers but on these I don't know how to apply it. Can someone explain how to use derivatives to find these answers?

1. Find the equation of the tangent line to the curve given by y=2xe^x. At (0,0).

2. Find the exact points (x,y) on the curve y=x ln x where the tangent line is horizontal.

3. Find the exact point (x,y) on the curve y= sin^2 x, 0<x<pie where the tangent is horizontal.