Mose people feel uncomfortable being photographed, but today with social media its unavoidable. With practice it will get much easier.
Practice with a variety of smiles.
Practice with big smiles, half smiles and closed mouth smiles to see what you like the best.
Communicate with your eyes.
Your eyes are the window to your soul.
Relax and breathe.
Focus on relaxing your shoulders, and do some light stretching before the shoot.
Keep your back straight.
A straight back shows confidence.
Avoid "Straight to Body" camera position.
Shift weight to your back foot and face the camera at a 45 degree angle.
Lower your chin but pretend there is an orange between your chin and neck. This will eliminate the double chin.
Practice hand placement.
Keep your hands relaxed. Hands on waist should have fingers upward and hands in pockets should have thumbs showing.