By pressing the spacebar you'll be able to select from the different types of gear you're carrying, and you'll also be able to change your firing modes.  The number of rounds and magazines remaining are also displayed.  Be sure to reload before you run out of ammo, or in any quiet moments - to save yourself from embarrassing moments :).

For an example of a weapon with a choice of firing modes, let's look at the M16.  This can be fired via single shots (tap fire button for each bullet) or burst fire which fires 3 bullets at once.  Burst fire is usually better because it will down a player once they're hit, whereas single shots will require you to hit the target 2 - 3 times (not easy when they start running!).

Although not a selectable option, I'd recommend "Double-tapping" a target.   When firing, fire off 2 shots quickly.  Re-aim, then repeat. This will ensure a higher hit rate and saves the hassle of missing the target if you rapidly hit the fire button (most shots will miss which also means less ammo available for the next target).

Probably the hardest trick to learn is aiming ahead of a moving target.   Bullets take time to travel from the gun to a target, so you will need to anticipate the position of your intended victim.  

Aim ahead to ensure the bullets hit the mark as the enemy soldier crosses their path. To get the range, let off one shot just ahead of the target. Once a bullet connects you'll then know the distance you need to aim ahead, and you can down the target.  Since most enemy troops operate in squads, once you've got the range for one of them the others who are nearby will be at the same range.   As you've already worked out the distance you need to aim, you'll be able to make short work of them.

In the image left (which is a plan view of the image above it) you can see the two russian's are running ahead, so I had my crosshair just ahead of them to ensure when I fired a shot off it will hit the target.  If I aimed right at them my shots would miss due to the speed, direction, and distance they are from me.

Note. The enemy will be doing this as well, trying to aim ahead of you so it's always a good idea to zig zag, or change speed and direction every few seconds if you're under fire.

Pointing the crosshair at objects causes a distance indicator as well as the units name / troop type to be displayed.  It will also change if you select any troops (More about this can be found here in the command section).

1 - The line in the middle will move as you run, and when you fire. Only when the line is within the crosshair will you hit your target.  So, unlike other games, you'll have to stop to fire as you would in real life. Crouching or lying down will steady the crosshair.
2 - When you're reloading, or for any reason you are unable to fire, this crosshair will appear.
3 - You'll use this one for grenades.  Now - basic grenades will have this circle and the dot appearing directly over it.   Stand still and point the circle over the area you want to lob the grenade.  It has a limited range.  To aim other grenade types like smoke grenades you'll see the dot within the circle when you're still.   This means it will land on target.

You can zoom in slightly by holding down the right mouse button as well. 

To conserve ammunition and dispatch a target quickly aim for the head.   Depending on the weapon you'll need to hit a target usually 3 times, but with head shots it only takes 1-2 hits. Sometimes it's hard to aim at the head (especially if the soldier is moving), so try to aim at the chest area instead.  A leg shot might cause the target to drop, but he will still be alive and will be harder to hit, so try to avoid this if possible. 

Depending on how severe the wound caused, a soldier can be forced to crawl (if his legs are shot up badly), or be unable to aim properly due to his gun arm having a gaping wound in it (crosshair will not remain stationary).  This affects you as well as the opposition.  However you'll only notice this on the odd occasion, because most likely when you're hit you'll be killed out-right.

As shown in the image left.  Pressing V will take you to the weapon view where you'll be looking straight down the sights of your weapon.  This provides the most accurate method of firing, although it restricts the view around you.

If the target is very close, you may find it helpful to drop to one knee and use the limited zoom  (right mouse button) to aim.  This will work fine and enable you to fire off a few shots, then move.  You never want to stay in the same place too long, or the enemy will outflank you.

Weapons such as missile launchers and sniper rifles will need to use this view to ensure pinpoint accuracy. 

You can pick up any weapon in the game as well as items of equipment and ammunition left by dead soldiers.  To do this, ensure you are near a body and enter the action menu (enter key or middle mouse button).  A number of options will be available.  This is useful if your only  anti-tank soldier has been killed and a enemy tank is fast approaching, or if you run out of ammunition for your gun, (which can happen a lot), in which case you'll need to grab some more, or pick up another weapon.

You can also rearm at weapon crates which are sometimes located on the map.