I want to install ceiling tiles in a drop ceiling. The grid is already installed. I have the tiles on hand - typical fibrous Armstrong ceiling tiles. Many of the tiles will have to be cut to fit the grid along the walls and to install recessed lights. How do I cut these tiles?

I have a garage that is 12x20. I am trying to turn it into an extra room for my birds. We stapled plastic to the ceiling this winter and put insualtion on top of the plastic. This kept the garage nice and cozy this winter. The plastic held up the insulation. I am now trying to drywall the ceiling. This makes my second attempt. I did it wrong the first time and had to tear it all out. I am now trying it again. I have a problem however. My ceiling joist are 24 on center. I have hung 1/2 inch drywall. I have hung it perpendicular to the joist. I have a friend that is telling me that my ceiling will sag in a year because I hung 1/2 instead of 5/8th. Is this true? The actual insulation is being held up by the plastic underneath. The garage has no moisture problems. Will my ceiling sag in a year? Is there anything at this point that I can do to reinforce the ceiling? Does 1/8 inch really make that much difference? I hope it won't sag... I hate to have to tear this drywall out again, rent another drywall jack and put it up again. The 1/2 inch drywall was so heavy I barely got it on the jack.. I don't know if I could lift 5/8th on the jack. I am a 5' 4" female and the drywall weighs as much as I do almost. I got no one to help me with this... I have to do it myself. I have attached some pictures. Please Help! I need some advise.... Thanks, Shawn