When proving two triangles are congruent, you use information and postulates you already know to create a logical trail from what you know to what you want to show. This tutorial shows an example of using a congruence postulate to show two triangles are congruent!
What is the Angle-Side-Angle postulate? This postulate is just one of many postulates you can use to prove two triangles are congruent! This tutorial explains the ASA postulate.