After choosing your starting location, or having it assigned to you, the game will begin. Some opening moves for each of the four factions are detailed below.
US, Great Britain, or Germany
You will start with one infantry squad (Germany) or two HQ Squads (US and Great Britain). These groups should be immediately be sent out to capture nearby flags. When a flag is being captured, an ever-expanding circle of your team color will start to expand from the flagpole in the center of the strategic point. Once this reaches the edge of the strategic point, your faction flag will be raised, and you will have captured the point. This will contribute command resources that can be spent to build structures, train units, deploy minefields, etc. The economic side of the game will be detailed later.
Meanwhile, you should click on your engineer (icon of a wrench can be seen when zoomed out). Wait until one strategic point is captured, and see how much command income it adds. You start with +20 command at the beginning of the game. Do some rough mental math, and figure out how much command income you expect to have after a few points are captured. If you expect to add close to or greater than 20 more command income in the near future, you should immediately begin construction of a barracks. On high income maps, this can be very quick (first few minutes).
While you are considering what structures to build, you should also click on your HQ building. Do you want to speed up construction of your barracks, or send more troops to the front lines? Note that each additional engineer will consume 15 more of your command income, so do not build too many. You will not finish your barracks much faster, and you will deprive the front lines of some squads that could lend much needed support. Once you have built the number of engineers that you like, building HQ Squads and setting your build queue to "Repeat On" is often a good strategy, so that you will not have to keep such a close eye on your production. You can also set a way point from the HQ to send troops to a designated location. Using the "Fight" setting for this way point is often useful, so your troops will not just run into enemy forces and be slaughtered. These same settings can be used for all unit-producing structures.
The Soviet playing style is significantly different. You start with some troops, along with 2 truck-pulled AA guns, 2 truck-pulled AT guns, 2 transport trucks, and a truck that can deploy into a supply depot, as well as 3 commissars (pseudo-engineers).
You will start in one location, but you are by no means obligated to build your base there. On many maps there may be an important choke point or vital high-ground that, if taken, would project your power over a large part of the map. If you are quick enough, you can often deploy a field base in this very important position, and become a great annoyance to your opponent, early on. This is a gamble, however, as if your opponent is able to catch you while you are setting up, you may be easily swept aside, as you have no way of obtaining reinforcements until you construct a barracks.
The economic notes for the other three factions apply for this one, as well. However, unlike the other factions, your troops capture strategic points very slowly while your commissars capture them very fast. Given the mobility afforded by the two transport trucks with which you start the game, you can easily transport a commissars to far parts of the map, and quickly capture many points. Make sure that you start building a barracks in a strategically important location as quickly as possible, or you will be quickly overwhelmed by hordes of enemy troops that will surround you.