A few questions, please help?!

Hi, my name is Adam and I am fairly new to using weights. I'm a SKINNY, 6'1 at 140 lbs. I obviously don't have much mass which is what I am trying to gain. I came across some dumbbells and weights for free about 2 months ago and have been using them three times a week since with ab work between days. As I said, I'm pretty new to this game and I have some questions on a few exercises with the dumbbells.
1) I feel like I'm not getting much contact with my upper-back while doing bent over rows.
2) I don't have a bench and have been doing floor chest presses. Same thing, I don't feel a whole lot going on for my chest and more for my arms. Should I continue to use this exercise, develop it, or try another one?
3) I'm doing dumbbell squats, but from what I've read squats effect the back as well. I'm just getting contact on my legs. I'm doing them with the dumbbells at my sides.
If anyone has any pointers for me I would really appreciate it. Again, my goal is to gain mass and weight, thus far these are the trouble areas I have been running into.
Thanks in advance
Adam