Start seated with dumbbells in your hands, bend the elbows at 90 degrees, engage your core & push the dumbbells up meeting at the top, then slowly lower back to starting position.

Beginner: 5-7lb dumbbells
Intermediate: 10-15lb dumbbells 
Advanced: 20-35+lb dumbbells

*Modification: You can also use a shoulder press machine or a fixed barbell