You need two partitions for Linux, a / (root), and a swap partition. The average sized swap is twice the size of available ram memory, but should not exceed 2GB for a home desktop environment. Your Linux should be right around what you allocated, 10GB. It can also work great on 8GB, but if you load it up heavy with tons of apps, you may be in a pinch for space later on.
Because you need two partitions, it is best to delete the current 10GB partition, then as stated in the prior post, during the partitioning step choose "custom", and then highlight the free space and select create, punch in the size of / (root) you need and label it as /. Then highlight the rest of the free space and create a swap partition.
At the boot loader stage of the installation, you may want to install the boot loader in the MBR of the first hard drive which would be "/dev/hda" for a IDE/ATA drive, for a SATA drive it would be "/dev/sda". Normally the installer will have the MBR of the first drive pre-selected by default. Then when you re-boot you will be presented with a menu to boot either one.