i bought a used macbook air and theres media thats taking up space from the previouse owner how do i clear it out i dont have the disk or anything just the computer

When you say there is 'media' taking up space, assume you know what it is? Movies, photo's, and the like? If you know what they are, click on each file and move it to the trash, then empty the trash.

If you just got the laptop, then chances are you dont have too many personal files on it yet... Which OS are you running... Snow leopard, Lion, or Mountain Lion... if you are running Lion or Mountain Lion, use the Recovery Partition to perform a clean reinstall of the opperating system, this will clear out everything the previous owner did.

If you have Snow Leopard or earlier, do like the last reply said and just delete all the old media files.

Reboot holding the command and r keys down, which will take you into the recovery menu. Wiping the disk clean will also delete any applications on the system, if that's what you want to do.

Wiping out the drive and installing fresh would definately fix that for you, but if you wan't to save the hassel a bit you could also create a new user in system preferences > users and groups then delete any old users, then go into the mac hd > users folder and delete any old home folders there

Do you know what model/year the machine is? And, which OS version is installed on it?

You need to figure that out because if it came with Lion or Mountain Lion, then the seller was supposed to transfer the license to you by calling Apple. If that's the case, you can easily use recovery to reinstall. If, however, Lion (or Mountain Lion) was purchased at the app store, the license is not transferable and you will not be able to ever reinstall the OS because the license is forever tied to the Apple ID used to purchase it. In that case, you will need to get the original install DVD by either calling the seller or Apple, erasing the hard drive and reinstalling the original OS. After that, you can upgrade to Lion (you need to pay for your own license).