If you are not sure about what to do, I'd recommend just follow the instructions of the installer and go with the partitioning suggested there, especially since you state yourself you are not an expert.
But, please make sure you have saved all your important data since partitioning or reformatting a drive will lead to data loss.
Messing about too much with partitioning is not a good idea on your machine anyway because Ubuntu has a minimum requirement of 15 GB (see the System Requirement page at help.ubuntu.com), so just go with what is suggested during the installation process.
So if you are confronted with a screen like this during the installation http://www.linuxbsdos.com/wp-content/uploads/2011/05/Uinstall1-600x413.png, go with one of the first three to options -- whichever you prefer and fits your needs.
You could also take a look at this guide to learn a little bit about partitioning http://www.linuxbsdos.com/2011/05/04/manual-disk-partitioning-guide-for-ubuntu-11-04/