Joining the shoulder is usually seaming the back and the front to each other over the stitches between the neck opening and the top of the sleeves. Once you've joined at the shoulder you can pick up stitches for the neckband as your pattern directs and later you can seam the sleeves to the body. Then you're left with a long seam from the wrist, along the sleeve to the armhole and down the side of the sweater to the bottom of the body.
The first diagram here
shows you one way to join the shoulder seam.
It's a good idea to pin the sweater together before you start seaming so that you can see where all the parts go and make sure they all fit well together. You're almost there and I'm sure you can do it!