Tax returns for 2012-13 must be filed online by 31st January 2014 and any tax and Class 4 NICs due paid by this date. If the tax due for 2013-14 is over £1,000 you will also have to pay 50% of the estimated tax due for 2014-15 by 31st January 2014 with the balancing 50% payable by 31st July 2014. These are called "Payment on Account" and will be set against the tax due for next year. If you earn less and therefore have less tax to pay you will get a refund and interest on the overpayment. If you earn more and have more tax due you will have to pay interest on the amount owing. You should therefore advise HMRC well in advance if you think you are going to earn at a higher rate than the current year or if you expect to earn at a lower rate you can ask for your Payment on Account to be reduced but you will be charged interest if you have then underpaid tax and may also be charged a penalty if HMRC believe you have not taken "reasonable care".
Penalties for not submitting your Tax Return by 31st January 2014 start at £100 and increase for each month of non-submission.
If you are not already registered to file your Tax Return online do it NOW. It can take up to 6 weeks to receive the authorisation code from HMRC.
If the majority of your income is taxed at source (you are employed rather than self-employed) you are unlikely to have to make Payments on Account.