Here is a quick step by step guide that I generally follow.
1. Open case and take out everything (in your case leave only the fans and the cables coming from the power switch, usb, firewire, etc.)
2. Find the motherboard standoffs (usually comes in a brass color) and place them accordingly in the case. You will usually use about nine of these.
3. If you do not have a modular power supply, I suggest you place your power supply in your case now. Trying to put it in with all your other hardware inside can be frustrating when you have cables all of the place.
4. Put your motherboard on a flat surface and mount the CPU and CPU cooler. Make sure you applied thermal paste and follow instructions provided by your CPU cooler's manual. Install your RAM by noticing the notches on the sticks and aligning it correctly in the DIMM slots.
5. Find your motherboard I/O shield plate in your motherboard box (a thin, rectangular, shiny piece of metal).
6. Place your motherboard inside your computer and use the correct screws to secure your motherboard in the case.
7. Insert your optical drive and hard drive as provided by your case manual.
8. Connect all power cables and data cables.
This includes, but not limited to:
24 pin ATX motherboard power
8 pin EPS cpu power
sata power for optical drives and hard drives
any 3 pin fans or molex fans
sata data cables
front power/reset/hdd led/ power led/speaker switches and cables
front usb/firewire/esata if applicable
9. Carefully seat your video card and connect the appropriate power cables.
10. Make sure everything is connected properly and power on your system.
11. Insert your operating system disk and install.
Hope this helps.