When I do "cfdisk /dev/sda" I get
FATAL ERROR: Bad logical partition 6: enlarged logical partitions overlap
Is this because I have Ubuntu already installed on the same HDD?
/dev/sda6 is a logical partiton for swap in Ubuntu.
Impossible to answer without more data, I'm sorry...
https://teksyndicate.com/forum/linux/quick-and-dirty-guide-arch-linux-installation/158212
Somewhere to start.
Try posting at the Arch Forums, too - much more qualified than I.
However, like Zoltan said, we need more details.