shoulder injury, weakness, clicking and grinding

Hi everyone, I've seen a couple of similar posts but still wondered if anyone could help me with my particular problem.

Both of my shoulders are fairly weak and pop and grind a lot but my right one in particular. I have injured it a few times playing rugby, the last time being about 3 years ago when I landed on my side and a few other people landed on top of me. I've only now just started working out seriously and notice that it is preventing me from doing a few things.

One of the problems is when I'm doing bench press with dumbells, and I'm in the position where my arms are holding the weights straight out above my chest, my right shoulder is so weak that it can't really hold it there and so my arm gives away. Another thing is that if I stand up and put my right arm out directly in front of me, shoulder height, palm down, my arm starts to shake and there's pain in my shoulder as it struggles to keep my arm in that simple position; the pain subsides somewhat if I rotate my arm 90 degrees (i.e., palm facing to the left).

I also get the same clicking and popping as many other people if I reach across the front of my body with either arm. I know my shoulders are very tight because if I put either arm behind my lower back and try to reach up my back I can barely do it. Theres no way I can do the thing where one hand reaches up the back and one hand goes over the head and the hands meet.

Sorry for the long post andi would really really appreciate any help.
Thanks