my laptops says i need to back-up to a disk what type of disk do i need ?

any, use a dvd

In order to get good advice explain your requirements better - for example state how much you expect to have to backup in one go and which optical media your laptop can write to. (The advice in the previous post is bad unless it can handle DVD media.) If the backups will fit onto optical media - be it CD or DVD - then consider -RW versions of that media if the backups need not be permanent. (So, you could re-use such rather than create landfill.) Consider also an external drive - always preferable since such are far quicker to write to and you can store several backups in one place given a suitably-sized one. Then there are online storage options such as Amazon's S3 - though before you seek more information on such you should specify your exact requirements.

Just get an external drive they are really cheap now.

Or you can get an 8GB Flash drive from picstop for £13.99

Sounds to me as if you bought a new laptop and wanting to make a backup of the system if so as dave650 stated DVD. Or CD but that will take several cds Writeables of course
You need to explain a little more if not or take some of the above advise.