Notices

Install backupmenu.
Turn your n900 off.
Open the keyboard, keep it open, turn your n900 on
Use the menu to make a backup of both rootfs and optfs on your sdcard.
When you send your n900 to nokia care, keep your sdcard home.
When it's back and reflashed, reinstall backupmenu.
Turn off, install sdcard, open keyboard, turn on.
Choose restore from menu, choose your rootfs and optfs from sdcard.
Wait until done.

hey thank you for you reply. but i dont have an sdcard. i wanna make a backup on emmc. after creating the backup can i store it on pc? and then when my phone comes back, i reinstall backupmenu and store the backup file from pc to phone and the restore the backup? is this possible?
lastly will this do any damage to the phone?
thnaks

You might as well make the backup on emmc then copy the whole directory "systembackups" on your pc. When your n900 is back, restore it where it was and proceed as detailed before.
Phone shouldn't be damaged by that.

ok u mean i can copy it to pc and then phone comes back, i will paste it back in the phone? then download and install backupmenu then restore?

Yes. This will restore all apps and settings.
But won't backup/restore your data in MyDocs partition. For that, you have to manually backup/restore all files through your usb cable (or via local network). Don't forget to backup also the hidden directories like .documents, .video, .music and .images and maybe some others.

hey im not getting what it what u mean. it backs up everything right? then why not mydocs? everything the sound and everything is also backed up right? i dont get what is backed up by this program

will this back up music, messages, applications, contacts and settings? and what is it that it wont backup?

There are several partitions in an n900. Those you're interested in backing up are rootfs, optfs and MyDocs.
The third partition, MyDocs, is a VFAT partition that contains your own data and is easily readable/writable with the usb cable, so it doesn't need any particular program to backup/restore: just attach your cable and copy. Generally speaking, MyDocs has all your music and photos and videos and documents. It is the partition that when you connect your usb cable should get the name "N900".

Rootfs and optfs, on the other hand, have peculiarities. Rootfs contains your operating system files; optfs contains other operating system files, plus your configuration files. They are not directly reachable through your usb connection, and are formatted in more "exotic" styles. They aren't easy to copy with a pc, and if you try to you won't be able to properly restore all the bits and pieces. For this reason, our friend RobbieThe1st gave us this wonderful backupmenu program which stores a copy of them and can copy it back without hassles.

ok so will this also backup my contacts and coversations? for the files in my memory like songs and pictures, i should use the usb normally. for my contacts, settings, conversations and all of the applications and widgets that i have installed, i should use backupmenu?

Yes, and yes.