Use vectors. This is dead simple and exactly how real games do this.
If your player is at position 10, 10 (Presume this is 2D space) and your enemy was at 20, 10. The vector from the player to the enemy is:
Player - Enemy
or
(10, 10) - (20, 10) = -10, 0
Normalising this vector gives you the unit vector, or direction. Which in this case is -1, 0. Multiple the unit vector (which is the direction remember) by a scalar value each frame and the enemy will move towards the player. What happens when it gets there is up to you ;)
