Step 1 is obviously to get a starter pack. But that won’t be enough, so buy some other packs as well. Whatever items you get, you get.
Each unit is worth a certain amount of points. The more powerful the unit, the more points it’s worth. Some of your units will contribute toward your Allied total, some toward the Axis total.
When you play a match, you want each side to have to choose their units. If for example you own 100 points of Axis units and 100 points of Allied units, allow each side to spend significantly less than 100 points for the match. If the Allied force consists of mostly American units, with some British and French thrown in, that’s fine. It’s also fine if the Axis force is mostly Japanese, with a few Germans or even Italians mixed in.
The problem you’ll run into is that some of America’s late war units are OP. This unbalances the game and makes it less enjoyable. To avoid this problem, stick with buying early war units if you can.