Select a good AC to start with and then get to at least Chapter 3. Start 
fighting the sim ACs in the ACSIS menu. A good amount of money is given for each 
fight, and as a bonus you get the defeated AC's emblem and schematic. You can 
use the defeated AC schematics to defeat more ACs because you only need a 
buildable AC to start a mission, and not a sim. The one drawback is that you can 
only use AC schematics that can be tuned to a certain level. If you have enough 
points to tune the AC up then you can use it, otherwise it will be highlighted 
in red instead of yellow or white. You can also use schematics earned in the sim 
online.