As stated, mkfs commands just do the fs layout stuff and not the partitioning. In order to do partitioning I'd either use fdisk or gparted. For fdisk, (off the top of my head);
sudo fdisk /dev/sda
p
d 1
(Repeat until there are no listings)
n p 1 (accept defaults)
p (you should see one linux native partition taking up all of the space)
I don't recall if you need to change the partition (t)ype or what you change it to.
After your done with fdisk, (w)rite the table and then run your mkfs command (which, I suspect if you look mkdosfs is actually a symlink or alias to mkfs.vfat