At this point you've just finished combat training and have been instructed to head over to the factory in D2.

The factory is where you can buy and sell units. You can have up to 48 units (you can increase the maximum number of units you can hold through reincarnation however).

First things first, sell the units they give you to start with. Nothing shouts "noob" more than having units named "Squad1", "Squad2", and "Squad3". Note: It's possible to rename units in the Lab or from the Item Equip screen, but for now lets just sell them and buy ones we want.

There are four types of units for you to choose from:
Infantry - Ground based humanoid type units.
Mobile - Ground based vehicle type units.
Aviation - Air based vehicle type units.
Organic - Ground and Air alien units.

As units from a division level up you gain exp for that division. For example, as you level up Imps, an infantry type unit, you hero gains exp in their infantry division.

When your hero gets enough exp in a division they gain a level in that division, which in turn earns them a stat point to be spent on one of the stats (tact, clout, edu, ma).

Therefore, when selecting which units to get you should have at least 1 set of units from each division.

When you reinforce a battle, you select which units you want to bring in. The maximum number of units you can bring into a battle at a time is determined by your Tactics stat. To be fully effective you should bring in only the same type of unit (eg. Imps) at a time. If you have the tact to bring in 10 units, and you bring 10 Imps to a battle, then it's a set of Imps.

When deciding what units to buy, you should buy them in sets. If you started with at least 20 tact, you can bring 7 units into a battle at a time (the initial 6 plus an extra one for each multiple of 20 tactics you have). That doesn't mean you should buy just 7 units for your set however. If your planning on going for more than 20 tact you should buy the extra units now so they can be leveled together, otherwise you'll end up having some units at level 23, and some at 13, which is just a mess, as they won't be able to equip the same equipment.

My suggestion is to get sets of 10 units each (80 tactics). If you level 10 units from level 1 to level 60 (max level for a unit) then your hero will recieve enough experience to reach level 50 (the last level you get stat points from). Plus, having 80 tactics will still give you plenty of points to spare to play around with the other stats.

You may have noticed a lot of units are grayed out. This is because you don't have the Influence required to buy them yet.

Influence is calculated based on your hero's level in that division and half their clout. Here's the calculation:
influence = division_level + clout / 2

Don't worry about having enough clout here. You'll get enough infuence just from leveling up your divisions to buy all the units you want.

So now you know you need at least 1 set from each division and 10 units in each set.

It's time to pick some units to play with. The following are the units you can select at the beginning that will be the easiest to level:
Infantry: Shade, Ghast, Imp*
Mobile: Pegasus, Behemoth*, Manticore*
Aviation: Hawk, Pelican
Organic: Ubik, Oizy, Poda, Larva, Mamos
*units marked with a star have slow movement speed.

At this point you should have 4 sets of 10, 40 units. Which means you have 8 spots left unfilled. There are some great support units to buy:
Red-Eye : Scans the map for enemies, and can equip a jammer to disable the enemy's vision.
Sapper : Can set mines for you enemies to run over and clear the mines of your enemies.

Note: There are more support units avilable such as the spirit to lock down enemy biological and mechanical units but they cannot be bought until you have higher influence.