There is no such thing as stupid question, nor is it a bad thing to be a beginner with computers. We all started somewhere.
6-32 screws and M3 screws are used throughout the computer.
These have been the universal sizes used for years. Your old computer has plenty of them. I suggest you just use the Philips head ones, and avoid the flat tip ones, (IF it has any. They do use them in the Older computers)
The flat tip screw head is a pain in the keister!
Here is an article on Wikipedia.org that has information on computer screws, and show photos,
http://en.wikipedia.org/wiki/Computer_case_screws
It's best to use all the mounting holes, but not a necessity.
The best method used to assemble a computer, TO ME, is to install the processor, heatsink/fancombo, and ram memory, BEFORE installing the motherboard to the Support Plate.
This way you can get your fingers under the mobo, (Motherboard), and support it when inserting the ram sticks.
You can also look at eye level across the mobo, and be assured that the heatsink/fan combo, is sitting flat on the top of the processor case. You have a teeny gap, you have Problems!
USE thermal paste!
If the motherboard manual says to use fiber washers, USE a fiber washer under the screw head, and against the motherboard mounting hole.
Rule of Thumb, is that if the mobo mounting hole has a metal ring around it, no fiber washer is used. No metal ring? Fiber washer.
MAKE SURE that there -> ISN'T a brass standoff on the support plate, that doesn't line up with a mounting hole in the mobo. If there is just one standoff in the wrong place, (Doesn't line up with a mobo hole), it can touch the solder joints on the bottom of the mobo, and short out the mobo! (Can short out the processor, ram memory, graphics card, and peripherals connected to the mobo, also!)