Walls and Ceilings - How to use drywall screwgun??

I'm about to do my first drywalling job. I actually did one sheet with my drill then I realized what a PIA it is to countersink the screw but not rip the paper. So, I bought the DeWalt DW257 on Ebay for $47. It didn't come with a manual (and there isn't one online) and to be honest, I'm not really sure how to use it. I don't even see a chuck to change the bit. I realize its probably a keyless chuck but I don't want to just start twisting!

Anybody have advise on how to use these screw guns? Also, I'm hoping this tool will eliminate most of this issue, but it has been very hard for me in the past (with drills) to keep the bit from coming loose from the screwhead and stripping the screw and bit! Is there any way to keep this to a minimum? Maybe a type of bit I should buy the doesn't strip so easily? I should mention, a lot of my house is made from 200 year old Oak beams.

Thanks again! This forum has been great!

Here is a link to the model I purchased, in case it helps http://www.dewalt.com/us/products/tool_detail.asp?productID=54

Thats the same one I use almost everyday. You get Number 2 drywall bits and just stick them into the end. You can set the depth of the screw by twisting the cone end. There will be a little diagram on there to tell you which way to turn. Once you have that to the correct depth you are ready to go. This gun has a clutch so when you hit the right depth it stops screwing. Old wood is pretty hard. You may want to try some fine thread screws go in better. Use 1 1/4". It takes a little while to get the hang of it. You have to keep it nice and straight or your screw will go in crooked.