I am new to RDD… Will i lose the reddcoins in my wallet if for example my computer crashes and i need to reinstall my operating system?
in your example yes you will but the best option is to back up your wallet (this can be done from the file menu in the wallet) to a USB for safe keeping. That way if anything happens to your machine you can reinstall the OS then the wallet and then restore your wallet from the back up
Can i also make a second backup on writeable media as a DVD for example?
sure just write the backup file to the disc