You need to first be logged in as root.
Type: "fdisk hda"
If you type "p" it will list your partitions on this drive, "m" will give you a list of options you can perform.
Now that you have your list of partitions open, type "d" to delete, and then the corresponding number of the partition you want to delete.
Continue this procedure to delete all the partitions you do not need.
Once finished, type "w" to actually perform the operations and write the partition table.