I just have a small question, my nokia n9 is 16 gb, but when i connect it says only 8 gb available, whats happening? is there any way to solve this? thanks

There is a program called Drop Cache that appears to help with memory problems, but you haven't mentioned any actual problems. I wouldn't worry about it, but others might have different ideas.

There's nothing wrong with your N9. The only partition visible through USB (when you don't use developer tools such as the SDK or flasher) is MyDocs (seen by the Harmattan OS as /dev/mmcblk0p1). Also, it's the only one formatted with the FAT32 file system, so any OS can access it (Windows can't read EXT4 for example).
If you wanted, you could play a game () with /etc/mtab, mount and umount, but one would have to suggest treading a wary path in this case.

Thanks a lot, but ijust have one more doubt, will i be able to use only 8 gb to keep things in my phone? or will i be able to install and keep things up to 15? thanks

If you go Settings -> Device -> Data Storage....
You'll see that there's a few places where your storage has been partitioned:
Installed apps 4.2Gb
Application data 2.1GB
User data 8-10GB. (on 64GB it says 57.7GB so 6.3Gb - 6.6Gb set aside)

So you should lose 6.3GB because it's already been set aside for the OS/apps/user data to be stored, the rest is yours for storage like music/videos etc, that MyDocs folder essentially.
I don't believe there's a single phone on the market advertised at 16Gb, where you actually have the full 16Gb available for storage, although the numbers vary by OS etc.

I believe there's ways to change the partition sizes but I'd personally stay away from that jazz.

You will only be able to store your own files up to what the Application Data says is available, i.e. 8GB. However this space does not include apps which is on a different partition.