Sorry, I did not recognize you are a girl. Be very careful while your doing it.
1. Disconnect the power first,
2. Discharge the static electricity on your body while working on the motherboard,
3. It's tricky to take off the blow fan on top of the radiator. Sometime you may need to take off some tighten screws. Don't (don't need to anyway) use much force to do it.
4. While your cleaning the interior of a computer, a vaccum cleaner is helpful. But be careful for all the wires and cables there.
If you don't know how to do it clearly. Ask a boy around to help you out.