Pick a good AC to start with andget to at least Chapter 3. Start fighting the sim ACs in the ACSIS menu. A decent amount of money is given out per 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 since you only need a buildable AC to start a mission, and not a sim. The only drawback is that you can only use AC schematics that can be tuned to a specific 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.