Can you help me? I want to make my flashdisk write protected. So no one can format my flashdisk because the data inside the flashdisk is important.
You cannot do that. At best, you can remove write permissions from files (at a filesystem level), so you cannot accidentally remove or overwrite the file.
I haven't seen write-protected USB Flash drives, but media like a SD card have such a protection switch. If the data is really important, make copies (backups) of the data and/ or store it on a SD card. After saving the changes and safely removing the media from your computer (Eject / Unmount), toggle the switch to make it read-only.
If you dont want someone to format your flashdisk then dont give them access to it. Even if your flashdisk has a hardware switch for write protection like SD-Card has, any one can easily use it to turn-off write protection. So your best bet is to keep your flash safe in your drawer and lock the drawer.
I would suggest you get your self some cloud storage and keep you important files there. There are free services available like DropBox.
And just for sake of information, here is a list of flashdrives with physical write protection buttons http://www.fencepost.net/2010/03/usb-flash-drives-with-hardware-write-protection/