You should receive a V11 letter a few weeks before your existing tax is due to run out. If you've moved and not let us know, or are a new keeper you'll need to use one of the other methods below.
You should have received your V5C vehicle registration certificate shortly after acquiring your vehicle.
You should have received your V5C/2 new keeper supplement (green slip) when you acquired the vehicle.
Get a V5C vehicle registration certificate (log book)
If you have lost the V5C with your name on, you can get a replacement by filling in a V62 form.