Tax your car online via the website.

Do not apply for your tax tisc before the 15th of the month (before its due to start)

Documents you will need:
v11 (your reminder letter - top right)
you can still tax your car without this document, enter your car details etc, manually. (ie car reg)
v5c (your log book - the document number, halfway down on the right hand side)

You can also make a SORN: ie, if your vehicle is off the road

 https://www.direct.gov.uk/taxdisc (Opens in new window)