If this is your first time with linux I personally would recommend using the 32bit CD and not the 64 bit. Not everything works well in the 64 bit version..
you better do some reading about partitioning the hard drive
eg
Partitioning the hard disk The installer will detect your hard disks and other hardware before starting the partitioner. This should only take a short while.
Partition disks You will be asked how you would like to partition your disks. Be very careful at this stage - if you partition your disks incorrectly, you could lose or erase data stored on your hard disk.
If you would like Ubuntu to automatically partition the hard disk for you, select Guided - use entire disk and press Return. This will erase your entire hard disk and install Ubuntu onto it.
If you would like to manually partition your hard disk, select Manual and press Return.
Partition your hard disks as appropriate, by creating and deleting partitions as appropriate. Select Help on partitioning on the main partitioning screen if you are unsure of anything.
Once you have finished partitioning, a confirmation screen will be displayed, asking if you would like to write the changes to disk. To continue with the installation, select Yes and press Return. Pay careful attention to this step of the installation process, you cannot undo changes made at this point, and you may lose data if you chose the wrong options. .. It might be an idea to have a few different partitions (better read up on these, because there are also logical partitions and extended partitions, and linux swap partitions), so that you can install, and try a few different systems. Also a good idea, to leave some space, in which to install a windows system. from memory,, this used to be the first primary partition
