To fly at constant altitude in a constant direction, your wings must be level. Check this by looking from the front of the cockpit to the horizon. If your wings are level, then the horizon will be flat. Next, check your VSI. This is the scale in the top right corner of the Head-Up Display (HUD). The line extending up or down from the zero mark shows the change in altitude in feet per second. The box to the left shows the exact value as a number.
To fly at a constant height, you want this reading to be about zero. If it is negative, pull gently back on the stick (or mouse), or tap the 'down arrow' key. You will see the horizon appear to fall as the aircraft's nose rises. How fast this change occurs will depend on how far you moved the joystick (or mouse) or how long you held the key down, and on whether you were in cruise, combat or landing mode. If your VSI has a positive reading, then the same applies, but push the joystick (or mouse) forward, or press the 'up arrow' key. "
From the manual