One way to approach this is to use the multiplication principle: if there are n choices for one thing and m choices for another thing, then there are n times m choices for both things together. In this case, there are three choices for a shirt and four choices for a pair of pants, so the number of outfits is 3 times 4, which is 12. 
You have three shirts and four pairs of pants.  How many outfits consisting of one shirt and one pair of pants can you make?