The highest and the fastest you are, the longer the range of your missiles. The angle of your target matters too.
Against su27 AI anyway it's really easy to win in a head vs head.
When you launch the 120 in your HUD (bottom) will appear a timer. At first you'll see the letter "T" followed by a number, wich is the seconds until the missile will start to track the target with his own radar.
Once the missile starts to track the target on his own, the T will become a M, followed by the number of seconds until the expected hit.
The smaller the distance between you and your target, the faster the T will become M. At very short distance the missile is active as soon as you launch it and you'll see only the M.
So the best way is to track the target with your radar until the T becomes an M. Once the T becomes an M you can break the lock.
If you break the lock before T expires, then the missile will turn on its radar and lock the nearest target.
If the AI shoots at you before the T becomes M, you can turn left or right until the target is at the very edge of your radar cone. If it shoots after the T becomes M, then you can crank even more, becouse you dont' need to keep tracking the target.
Anyway, when they shoot from far (i mean the AI... not humans) you can just slow down and keep going straight... the 120 will be active (the T will be M) before the 27 will reach you. Once the 120 is active infact the target will engage defensive and break the lock on you... so the 27 will go balistic.
A human will be smart enough to go defensive without breaking the lock...
I suggest you to read the manual anyway. There you'll find the basics.