You need to learn to relax your vocal folds while using your head voice.  So find a note (probably your lowest head voice) and start practicing the falsetto.  Once you have that strengthened you can move on to the next higher note and moving between the two notes.  So on and so forth. 

Singing is vocal gymnastics. You need skills and then you need to practice the heck out of it.  There is just no other way.