As stated here in
this other post you can only view who sent you reputation points
if you are a contributing member to Fauna Classifieds. Otherwise you are stuck with only knowing how many points (- or +) you have.
The explanations have been put in the above link as to where to find the points and comments, etc.
As for how you leave them - when you see a post from a person you think makes sense, or from someone you think always has meaningful things to say click on the little reputation icon near the bottom left of the post. It sort of looks like a bullhorn/speaker thing. Click on it, enter either positive or negative (if you think they never have anything to add to the conversation and are just blowing smoke up everyone's butt) and then enter your comment.
Hope this helps,