If you were able to tie your shoe before you got to the age you are in, then you are developing a coordination problem between your brain and other body part movements. For that you should seek medical intervention on. In the other way if you had not tried in the past then it is the high time you tell your friends to start teaching you, and with time you will understand.
Okay my dad was in the military we are DEFINITE experts at this, you cross the laces, and you take the right lace and pull it underneath the hole, next you make a little circle with the right lace , now pull your pointer finger out and there will be a little hole, put the other lace half way in it and pull on both laces hopefully these directions make sense! have fun!
I believe my first question would be, is this the only coordination problem you have? If not I would seek medical advice as to why you have such a problem with eye/hand coordination. If this is the only problem you have, then have your parents or friend sit with you and go thru the motion until you get it down.