Just started lifting last week

I just had a couple questions for some advice.

My muscles still feel tight and especially so on my left side of my upper body in back and tri's, I expect it has something to do with me being right handed. It's quite painful to stretch out too much in my left tri and back, do you think I pulled a muscle? Also any stretches to recommend or general advice on how to get rid of this stiffness more quickly before the next week's workouts?

Thanks.