If you are self-employed then you will have to ensure that you file you tax return by the 31st January 2016 for the tax year April 2014- April 2015. You may have already done so, which is great, but if not this deadline must be adhered to in order to not receive a fine. This process is only available to people wishing to submit their tax return online. If you want to do a paper return then the deadline for this has already past.
Tax returns can be complicated and you may need to speak to an accountant to check that you are doing everything correctly. You may just want them to have a look over it all for you and check you have not missed anything or you may wish to hand them all your invoices and receipts and then ask for them to work out exactly what you need to pay. This method will of course cost more.