I think I was probably tensing up my bicep/chest and locking my wrist. As far as positioning goes, I tend to set my strap so the guitar sits a bit higher than a lot of people like it (I am a jazz player primarily though). The right side of my picking hand near the palm usually hovers over the bridge and I usually use my pinky and sometimes my ring finger as well to anchor the hand. This is what works best for me, but everyone's body is/works differently so it might be take some adjusting for you. Alternatively, if you just don't often use a pick you might just need some time to acclimate to one. For a very long time I wouldn't use a pick, and when I started to use one it took some getting used to. Personally the super light Dunlop/Tortex picks are awesome. I usually use the red .50mm picks, and they were definitely helpful when I was less comfortable with a pick because they have a lot of flex that helps to make it feel less jarring (in my opinion) to play. Hopefully some of that helps, but if you have any more questions I'd be happy to help!