Indian clubs have helped my shoulders. The clubs move your joints around without much weight and identify tight spots.
I wish I had used the clubs as a newbie.
If possible, try to exercise muscles that oppose the paddling stroke.
A balanced shoulder is a happy shoulder.
As your balance improves, you will be able to engage more of your core. That's where the real power is, but it takes time to link it all together.
As you transfer power from your feet to your hands, you will encounter weak links in the chain. The shoulder is a very common weak link. Don't abuse it.
There are heaps of information on the web. Asking now is very wise of you.
I usually go to Dave Kalama's waterman's journal to remind myself how to paddle correctly.
www.davidkalama.com/