You partition your HDD via Disk Utility into two partitions (add a second partition via the "+" icon under the PARTITIONS tab) and then choose that second partition as installation target during the installation, though you should have the installer on an external USB flash memory thumb drive or on a DVD.
Also think about upgrading your Mac to 4 to 8 GB of 204-pin DDR3 SO-DIMM RAM, 8 GB (two 4 GB modules) cost less than 50 USD via Newegg or Amazon or Crucial or Corsair.
You partition your HDD via Disk Utility into two partitions (add a second partition via the "+" icon under the PARTITIONS tab) and then choose that second partition as installation target during the installation, though you should have the installer on an external USB flash memory thumb drive or on a DVD.
Also think about upgrading your Mac to 4 to 8 GB of 204-pin DDR3 SO-DIMM RAM, 8 GB (two 4 GB modules) cost less than 50 USD via Newegg or Amazon or Crucial or Corsair.