Newbie here. Just started using Blender. I did look through many questions, but most likely don't know what I should look for. I watched tutorials, tried to fix vertex paint, vertex weight. starting to feel lost, can't seem to fix this! I would be so happy if someone could help me with this! Thanks!
You have not properly assigned weights to your lowerarm. Select this bone in Pose Mode, then select the character mesh and press Ctrl+Tab to enter the Weight Paint Mode. Set the Weight factor in a Tool Shelf (T) to 0.000. Now select every single bone of the hand and see if there are weights assigned to the rest of the body and remove them (paint them, so they're blue).
NOTE 1: I haven't removed all the weights- it's just an example.