I assume by your comments that you probably have ordinary plate glass (the stuff that looks green when you look into the edge?) and if it`s at least an inch thick then it would probably be a good piece of glass to do your first mirror. On the web there is a lot of sites that tell you how to do the calculations for the "focusing optics" and so forth. You might also want to drop by my website at http://webu.wigloo.com/bobmay/ for my own particular way of doing the job. You may also want to find the astronomy club in your area and see if there are any mirrorgrinding gurus laying about doing nothing.
A 14" mirror is a very nice size to own without being too big. Try to make the focal length as long as you can as this will allow for better views in the eyepiece.