Open Disk Utility & select the drive you want to install your OS on.
Open the Partitions tab & add 2 partitions by clicking on the
little "+", then arrange the partition size as required
Make sure to name your partitions so not to get mixed up later
Click apply and wait for your partitions to get created.
Run the installer & when it asks which disk you want to install; choose show all disks & pick the one you want
You probably have a bootable disk to install ubuntu. Insert the disk & restart your computer while pressing down "c" this will boot from CD. (It might take a long while to boot from CD). Run the ubuntu installer & when you are sued to choose a drive to install on choose accordingly.