Sideslip Indicator

   You sure know the little guy pictured above, right? That's called Sideslip Indicator, aka "The Ball" (except on British planes where it's replaced by a needle)
Basically, all you need to know about the sideslip ball boils down to a couple of rules:
- Use rudder to keep the ball centered
- "step on the ball"
In other words, push on the pedal on the same side where the ball goes. Ball goes left, apply left rudder. Ball goes right, apply right rudder. That's Pretty much it. But let's have a quick look at why you have to 'step on the ball', and what 'stepping on the ball' actually does.

   "Step On The Ball"

The ball is centered when the plane's longitudinal axis is aligned with the direction of travel






If the longitudinal axis is not alligned with the direction of travel, the ball in the sideslip indicator moves to one side






Apply rudder to the same side where the ball goes ("step on the ball")

    As you can see from the bottom picture in the previous section, applying left rudder pushes the plane's tail to the right and aligns the longitudinal axis with the direction of travel.
And at this point you're probably wondering what you're supposed to do in a turn, when the direction of travel changes.
The short answer is just keep the ball centered as you turn. You do that applying rudder to the same side as aileron, and that's called a 'coordinated turn'.
The long answer involves some basic physics (well, what you expected? Football?). Remember the old Newton rule stating that a body in motion tends to preserve motion? That's why you can throw a rock and it won't fall on your feet. OK, now think of a car, instead of a rock. You drive fast on a straight road, then you suddenly turn the wheel to one side without slowing down and without properly approaching the turn. What happens?
You got it: the car rolls over. Or at least tries to. but why does it do that?
Because you tried to make it change the direction of travel without dealing with the energy that tried to make it keep going straight

How does that apply to aircraft? Well, a plane is (kinda) comparable to a car running on ice. If you don't turn properly it will just keep going straight because the energy is greater than the wheels' grip on the surface, so it will just slide along instead of turning.
And that's exactly what happens if you apply rudder ("turning the wheel") without banking your wings ("properly approaching the turn"). Our plane will sideslip but it'll keep going essentially straight. And that's where our automotive analogy ends, as cars don't bank.

So what happens if we bank our wings but dont apply rudder to the same side?
The answer is that the plane will in fact turn, but not in a coordinated fashion. The direction of travel won't stay aligned with the longitudinal axis, which will expose more surface to drag, which in turn will increase the power needed to attain a certain speed, which eventually leads to loss of energy (speed/altitude) and increased fuel consumption.

   "Step On The Ball"

Wing bank forces the plane into a turn but its energy make it want to keep going straight






Apply rudder to the same side as the plane is turning (in this case, right)

As you have probably figured out from most of the lastest paragraphs, you can use your rudder to aim your guns: rudder pushes the tail around, aiming your nose to the same side of the pedal that you push.
Also fairly obvious (especially when someone tells you), the increased drag caused by sideslipping can be used at our advantage, for instance to "slam on the brake" and make our pursuer overshoot. Or if you're on final approach too high and/or too fast ("real men dont go around" :P ).
The technique is simple: lift one wing and apply rudder to the same side of the wing you lifted. That's called "crossing controls". Practice this trick, you will find it useful sooner or later.

   These are a few basics, there's a lot more to say, and most important a lot more that you can find out with practice, so stop reading and fire up that game right now.