In step 4 of the installation you should have several partition methods to select from;
1. Use entire disk - Formats entire disk in ext3 and installs Mint using all available space (easiest for first time users)
2. Resize partition and use freed space - shrinks existing partition and installs Mint in recovered space while automatically setting up a dual boot with existing OS (usually Windows)
3 Use largest contiguous free space - similar to 2 but requires space to be freed up before beginning installation.
4 Manual - Advanced partitioning method for more experienced users. allows user to set custom partitions for root, home, swap, media, etc.
Choose one of these by clicking the proper radio button and click Forward. Your choice will be implemented and partitioning will be done automatically (except for manual).
For a detailed discussion of installation and partitioning, read this: viewtopic.php?f=90&t=11872