The military press is a great move to hit most of your shoulder muscles. There are lots of variations, including those both sitting and standing, to meet your individual needs.
You can also adjust this exercise to meet your needs by using your stance to alter resistance: taking a wider stance will raise the resistance.
And one last note before you begin: Remember, back straight, abs tight. This will help keep strain off your lower back.
The Military Press
This exercise targets your front and medial deltoids and your traps.
Stabilize one band under each foot with the ends close to your heels. Set your arms up laterally and elbows bent at 90*.
Push up until your arms are extended above your head.
Lift one band at a time, moving arms in opposition to isolate each deltoid.
Seated: Fasten two bands on either side of you or wrap one band under a post.
Raise bands in a straight line behind your head to engage trap muscles.
Insert a bar to balance and lower overall resistance