in "dogfight" its every man for himself so if someone is on your 6 then the best thing to do is probably make peace with god....your gonig to be seeing him soon.
In "team battle" things are much easier, the BEST thing (i find) is to have a wingman. A good one helps
For example....
i fly the spit IX online (the XVI is good but not as good as the IX in a turn) and the majority of the time in team battle were up against 109's. So, our strong point is the turn.
Get someone on my 6 then i'll pull a tight turn, and keep it going, lowering speed helps because it tightens my turn and a rookie 109 pilot will try the same but wont be able to keep up, While he's to busy with me and trying to keep turning with me.... my wingman is now on his 6 and lining up an easy kill.
So from my point of view..best thing is a wingman. They have helped me out alot on this game and vice versa.
If no wingman...
then a mixture of everything you've been told above by the others
you will soon pick it up. It helps to know your aircraft though, know what its good at. And also know the weak spots of your enemy.