How do I strengthen my shoulder and make it more flexible?

[Mystikal]

For serving, the main thing holding me back is my shoulder starts to get really fatigued and a bit sore.

[tenniskid3119]

I dont know what its called but to like a hand stand against the wall, and do push ups that way. Like, litterally push yourself up and down.

[Bagumbawalla]

I would start with ordinary push-ups and pull-ups.

Also, try this- Lay on your back and push yourself up so you are supporting yourself with your hands and feet in a kind of upside-down crab position. Now, walk yourself across the room in that position.

If the above things are too easy, find a gym with gymnastics equipment and work out on the horozontal and parallel bars and the still rings (with instruction)- I'm surprised more tennis players don't do this.