If you already have a girlfriend just pray and let God guide you! Sometimes God will lead you to an unsaved person that He knows will come to know Him. Other times it is probably easier to look for a Christian.
The unequally yoked thing can relate to more then one level. For example, equally yoked people are pulling in the same direction with the same amount of force. Most ladiesnor men of the opposite sex, despite being Christians have different visions or goals in life and will not necessarily make the ideal husband or wife. (Hence why one needs to go courting for a while before marriage, and dont worry... You may go out with many before you find the one for you!)