Normals? Then how do I solve this?
Normals are vector that indicate the direction a triangle or quad is facing. In order to see them in Blender you should check the face normal button on the Display section of the Tools panel:
Normals should always point outward, or it will introduce strange lighting glitches in-game.
As Betrayal says, you can use the "Recalculate normals" or the "Flip normals" command from the Specials toolbox (the one you get with "W").
If this doesn't work, it may be that normals are getting messed up by negative scalings, so if your model has inverted normals only when you export, you should apply scale (Ctrl+A in object mode) and re-check normals.
