How to do missions

I did the intro missions but nothing else pops up I work in the military and I go to school but how else can I level up where do I find people to attack and stuff

So the next person you meet is Duke, He pops up after a few days when you're done with George, so give it some patience.

Duke's missions are unlocked when you reach level 5

you can search any random 3 letters like "xyz" and attack level 1 inactives (or highest level you can easily kill) from that list.  this will help you level up.

Missions aren't progression through the game, you just completed the tutorial. In Torn you need to find your own goals to pursue as the game itself doesn't give you any. Decide for yourself what it is you want. 

Here's a new players guide to get you on the right track https://www.torn.com/forums.php#!p=threads&f=61&t=15993234&b=0&a=0