Partitioning

Hi,

I want to install ubuntu 11.10, alongside with windows at the moment, because i'm not sure if i want to go all the way yet.
I got stopped at the partitioning step, where even without making any changes , i couldn't go further because I didn't define a root system ( ???) I don t really understand, and know what it means . Also, i would need recommendation and how to partition efficently a 320 Go HDD, for simple needs.

Thanks in advance

Kev