Targeting System (Looking for tutorial?)

Hey guys, so I seen a handful of tutorials out there about a target lock system, some are pretty neat but not very detailed while others are very detailed but not what I am looking for. So I turn to the forums! My idea is this:

I want a simple targeting system that when I press a button (TAB) the target switches to the nearest Enemy, NPC or Player. This will then display a circle around the Enemy, NPC or Player and if I run around the target, the display will rotate with me, constantly facing the screen. Also, I want the target display to change colors based on the level difference from the player. (EX: if the player is level 10 and the enemy is level 11, I want the circle to be yellow, but if the player and enemy are same level, then circle to be white.) I don’t expect anyone do do all this FOR me, But I am hoping maybe someone can point me into a direction with a tutorial or maybe help me via Skype or something. Thanks guys!

Anyone got any ideas?