We can be more helpful if you can let us know what data you'd like to back up.
Photos?
Music?
What model is your S2? What version of Android is it running? Rooted or unrooted?
There are a few different ways to back up, depending on what you are trying to save.
For your phone contacts, if they are all stored in phone memory, you can go to contacts-->menu-->import/export-->Export to SD Card. This will create a backup file of your contacts onto your SD card. I would highly recommend uploading this file to your google account and importing the contacts into your Google contacts. Any future contacts you save should be saved to this google account. The advantage of this method is you never have to worry about losing your contacts again.
If you're trying to back up your applications, all of your applications are already "backed up" in the sense that your google account keeps track of all of those apps. When you do a factory wipe or get a new phone, all you need to do is log into your google account, go to the play store, go to "My Apps" and the list will populate. You can download the apps from there.
If you are trying to save app data or game data, some games/apps allow you to backup or store to the cloud automatically, so you may not need to back that up.
As for pictures, music, etc. If you have those items already stored on the SD card, then you don't have to do anything to back those files up. Just be sure not to reformat the card and you won't lose anything.