Writing on NTFS
 

I did
sudo apt-get install ntfs-3g
and everything installed fine

but I still cant write on ntfs harddrives
I get
mkdir: cannot create directory `a': Read-only file system

you have to remount the drive as read and write

how?
I didnt do anything
it mounted the drive on its own

Try the -w switch with the mount command you are using to mount the partition.
Although -w is default behavior.

Type: fdisk -l in a terminal or look at /etc/fstab to find which device the ntfs partition is. For example:
If it is /dev/sdb1, and is accessible through the file system through /media/windows_c, then you un-mount it with command:
Then you mount it with ntfs-3g like so:
Then edit /etc/fstab to have it mounted with ntfs-3g every time you boot up by changing the line in /etc/fstab for /dev/sdb1 like so:
If your ntfs partition was not available before installing ntfs-3g, you have to make a directory for it in the file system, you can make it in /media, /mnt, or in root if you like and call it what you want, I'll call it "windows" for demonstration purposes, as in the example that follows:
Then you mount it with similar command from above and add a line in /etc/fstab similar to the example above.