How do i solve this circuit with Thevenin's?
Method1 (specific method for voltage divider):
After you simplified the parralel resistors \$R_{23} = R_2 || R_3\$ you got a voltage source with voltage divider formed by \$R_{23}\$ and \$R_1\$.

The Thevenin Equivalent of a voltage source \$V_{src}\$ with voltage divider is a Thevenin source with \$V_{th}=V_{src}\frac{R_{23}}{R_1+R_{23}}\$ and \$R_{th}= R_1 || R_{23}\$ 

Method2 (general method that works always):
If you don't know this shortcut above: do what you should have learned in the lessons: 


find open circuit voltage \$V_{oc}\$ between a and b
find short circuit current \$I_{sc}\$ between a and b
use both results to derive \$V_{th}=V_{oc}\$ and \$R_{th}=\frac{V_{oc}}{I_{sc}}\$.