First lets eliminate the obvious:
1. Is your gear up? (Key G)
2. Are your flaps up? (Keys Q down & W up)
3. Are you overloaded with bombs/rockets/drop tanks?
4. Are you at full throttle/full RPM? (verify throttle mapping in set-up... Keys +/- for RPM)
5. Is your stall limiter enabled? (disable it in set-up)
Now a few other questions:
Are you just taking off?
Are you trying to climb to the enemy?
If so you're too slow to manouver. Keep speed at a minimum of 200 in virtually any plane and preferably higher to engage.
Are you mis-matched? Are you trying to turn fight a NIK, Hurri, A6M etc with a 190, P-51D, etc? Don't turn fight with a B'n'Z plane and be careful B'n'Zing with a turn fighter. Know your plane and your enemy's plane and exploit your strengths against his weaknesses.
Beyond that many other things can come into play; fuel load, gun package (weight), use of rudder, flaps, throttle, etc.
As long as you've got the first 5 covered you're on your way. The rest just takes flight time and practice.
Hope that helps.
Have fun with it, I bet those first two kills felt pretty good.