Get yourself to a good yoga instructor learn some basic yoga poses and then once you have some body awareness and control you may be able to look into more specific poses.
Simply looking at pictures of some poses to try will not teach you about correct foundation, muscle engagement, extension etc. From your description it sounds like your shoulders need some opening and strengthening, along with core and back work.
Get yourself to a good yoga instructor learn some basic yoga poses and then once you have some body awareness and control you may be able to look into more specific poses.
Simply looking at pictures of some poses to try will not teach you about correct foundation, muscle engagement, extension etc. From your description it sounds like your shoulders need some opening and strengthening, along with core and back work.