[Daniel Brim]

I'm assuming you put a new sketch on a face. If you did it on the end, celebrate, because all you have to do is go to part features, click on extrude, click on the picture that indicates cutting away (it's the middle one), enter the distance you want to extrude, and you're done.

If you are doing it from the side, it's a little more complex but it isn't all that bad. On your new sketch, make a line that is coincident (touches) the top and bottom of what you want to extrude. Make sure the line is paralell to the end of the piece (or perpendicular to the lines it touches, it's the same thing). Then click on general dimension, and click on your new line (the green one) and the end of the piece that you want to extrude. On the dimension, enter the distance that you want to cut the piece back, and go to extrude. It will ask you to click a profile, so click what you want to extrude away. Click on the thing to extrude the piece away, and where the thing that says distance is, click on the arrow to the right of it and change it to all.

Good luck and HTH,
Daniel