The game uses a system of cards (freely available on the internet, but once upon a time we had only the cards from the core box and supplemental boxes) to create armies. There are four kinds of cards - core, support, special, and character. The core cards form the units, and the others are added on to it.
An army must have at least two cards, plus an additional core card for every 150 points above 300 (so two for 300 and below, three at 450, four at 600, and so on).
The army can have two support cards and one special card per core card. While the rules specify that each core card can have up to two support cards attached, there's no limit to the number of special cards that can be attached to a core card (other than the limit of how many in the army). Both of these add mecha or upgrade existing mecha.
Characters are added to existing mecha.
All of these cost points. If you're playing with just Wave 1, you won't want to go much outside the 300-450 points range. The problem is the Zentraedi either get overrun by the UEDF at too high a points, or get wiped out at too low.
As an aside, beware playing on too small a board or with too little terrain. The Zents need something blocking the UEDF's ranged firepower, and everyone wants room to maneuver. 4'x4' is good, but 4'x6' is better. The book suggests you cover at least a quarter of the board with terrain - I wouldn't go up over half, though.
