Practice yawning. This deep inhalation naturally lowers your Adam's apple and relaxes your throat muscles for singing.
Start on a comfortable note, such as middle C, and sing "gug" for each note up and down a major arpeggio four times. Making the "gug" sound continually forces your Adam's apple to remain low in your throat, regardless of how high the note you are hitting is on the musical scale.
Repeat the exercise, increasing a half-step each time. Do not force the notes. As your Adam's apple becomes accustomed to remaining low in your throat even during increasing scales, the notes will naturally thin themselves, requiring less air pressure and effort on your part.
Replace the "gug" sound with a "mum" for each note once you are comfortably singing "gug" at the top of your desired range without strain. Repeat the exercise using "meh" to replace the "mum" sound when it becomes easy to sing.