I need to find the altitude of the triangle 
13 by 10 by 13 
I have no idea how to do this.

Here's the triangle:
 


Draw in the altitude, a, which divides the base (which
measures 10) into two parts measuring 5 each:



So now you have two congruent right triangles.  I'll split 
them apart for clarity:



With the right triangle on the left we can
use the Pythagorean theorem to find a:







Subtract 25 from both sides





So the altitude of the original triangle is 12.

Edwin