Hello Lara, and welcome to Seven Forums.
You can use the tutorial below to show you how to do "Clean" (formated) install of Windows 7. When you get to steps 7 and 8, you can delete all listed partitions to completely wipe and format the hard drive.
Clean Install Windows 7
I would recommend that you double check to make sure that you have Windows 7
drivers for all of your devices first, and have then downloaded and save to say a DVD or USB key in case you need them after installing Windows 7.
You should also create a set pf Factory Restore/Recovery discs from your laptop in case you should ever decide to restore the original OS that came with your laptop back on it again.
Hope this helps,
Shawn