When you are applying to renew your car tax online (or road fund) you must abide by certain conditions that apply to both the car and the registered keeper (the registered keeper can also be the registered owner, but this is not always the case). You can renew your tax disc at a Post Office, but if you prefer not to queue up you can renew it online.
1 - If You Received a Reminder from the DVLA
With UK road tax renewal, generally, your V11 form will be posted approximately two or three weeks before the old one runs out. If it arrives with you sooner you can still apply for a renewal, but no earlier than the fifth date of the last month. If you are applying with your Vehicle Registration Certificate to renew your car tax in the UK, it is acceptable to renew your tax disc on the first day of the month of expiry. It will still only take effect on the first date of the new month and still expire either six months or twelve months after.
Insurance certificates are all now kept on the MID database so that records can be kept up to date. If you apply for your tax disc online you can pre-check the database before you apply. All MOT records are also on the MID database now, to ensure that all vehicles have been classed as roadworthy. If you are sure that you're in possession of the documents you need to apply online, you can do so.
2 - If You Did Not Receive a Reminder from the DVLA
You can apply with a V10 (Vehicle license application) if you have newly acquired the car but have yet to receive a V5C (log book) registration forms back from the DVLA yet.
If you do not have the registration documents you must be in ownership of the 'new keeper' appendage from the original V5 registration form. This is the 'sale or transfer' appendage on the bottom of the registration document that you keep when you purchase the car. It should be the red part on the document. You must also produce a valid insurance certificate, current on the date that you are purchasing the car tax disc.
You will need a current MOT certificate if the vehicle is more than three years old, and the MOT certificate must valid on the day the tax disc becomes valid. If your vehicle is less than three years old it does not yet require an MOT certificate.
In all cases you will need to have the proper sort of payment ready. You can pay with a valid debit card like Visa or MasterCard, but note that if you pay using a major credit card you will be charged a service fee of £2.50. Therefore, if you do opt to pay online, it's a good idea to use a debit card to keep the costs down a little.