Find equation of the tangent line to $y=x^2$ at the point $x=3$ 
Find equation of the tangent line to  $y=x^2$ at  the point $x=3$

I tried to use derivatives and I found out the derivative is $2x$. How to continue? 

You got it!  The derivative is $2x$.  So at $x=3$, the slope is $2*3=6$.
Now you have the slope, all you need is a point.  Your function is $f(x)=x^2$.  So at $x=3$, a point is $(3,3^2)$ or $(3,9)$.
Now you have a point and the slope.  Use the point slope formula to write the equation of your line.
