This game, like Zeonic Front, gets harder after gameplay. (In this game, each replayed level has much more mobile suits than the first time playing through, and some MS are stronger units than they were before)
The first time you are playing through, simply beat the first 9 missions. Don't beat mission 10, or else you'll finish the game.
Keep beating stage one or two, or whatever stage you can get an S rank on, using different MS. Eventually after 6 completed mission, you'll get all MS upgrade equipment, like the long range radar, increased booster time, and a few others.
Once you've gotten the Long Range radar, go ahead and complete mission 10. Next time you play, try to S rank stages in order from mission 1 to 14.
As you S rank, your Command Rank will go up, from Captain, and so on. Once you obtain a certain rank, you are able to unlock even more suits.
Yet sometimes you will need to play, at a certain Command Rank, with a certain MS to get another one. For Example, at a certain rank, you need to use the Ground Gundam to complete a few missions, to get the EZ8 Gundam, etc. Just make sure to use every suit at least 3 times. Have fun with it, as this gundam game isn't so hard to get all MS, and is better played while enjoying the game rather than trying over and over to get good scores.