As of 16th December 2013, you can tax your car online or over the phone, provided that you are the registered keeper and your name, address and any changes that you have made to your car have been updated with the DVLA.
It can take up to 7 days for your insurance information to reach the Motor Insurance Database if you have just taken out a policy. To check if your details have been updated, you can visit www.askmid.com/ownvehicle/ to check for free.
To tax your car online visit www.direct.gov.uk/taxdisc or call the DVLA on 0300 790 6802 to renew by phone.
Alternatively you can buy or renew your vehicle tax in person by visiting your local Post Office branch.