Tree Foo Boo

Finally installed lightroom. Don't really know what I am doing with it yet but had a friend pop over to show us the basics. I think it is going to revolutionise my photo filing if nothing else! So, you know that gradient tool thingy, top right, looks like a rectangle with lines coming out the side? How do I set it so I can see the lines on the photo as I drag the mouse accross? At the moment, it's invisible so it feels like guess work. Also, how do I get the circle thing (is that part of the same tool?) so i can lighten just a face, for example? Friend did show me all this but it seems to have exited my brain. I'll need to do it a few times before it sticks!