You need to add some more info about what you're doing if anyone is going to be able to give you any advice. Are you pitching or yawing? You should be using roll to get the target directly in front/back of you, pitch to keep him in your sights and yaw to fine-tune your aim. I like to add downward thrust while pitching upward with the throttle in the blue. This will give you the fastest turnrate possible while doing somewhat of a drift because of the downward thrust. If I'm forced into pitching down then I would counter that with upward thrust during the pitch. This is more for a circle-fight than a jousting run.
If you like to lance or joust you should be boosting past the enemy, turning FA off, doing your pitch-flip, turn FA back on, and make another run.
Also, the way you distribute power can affect your turn rate. If you put some pips into ENGine, you'll turn a bit faster.
Combat is hard to describe with words, I suggest you look at some video tutorials and practice.