When you first start your pc, there should be a set up key, it is usually f2. so push the power button and just start tapping. Your pc might beep at you but nothing bad will happen. This should take you into the bios.
From there, you usually navigate with the arrow keys but there should be quick info on the side or bottom letting you know what does what.
Compatibility mode is accessed by RIGHT clicking on the program icon and left clicking properties. there should a tab that says compatibility. Go to that tab and then you can select the OS style you want and there are a couple of other options but usually os choice is enough.