The deadline for filing paper tax returns was 31 October, and if you have missed it then you would do best to see an accountant, who can file an electronic tax return by the electronic deadline of 31 January 2018.
You could try filing your own electronic tax return, but messing about with the software is not a sensible way to spend Christmas, and as your own tax clerk you already deserve to get the sack for leaving things this long.
The priority is to get a Unique Taxpayer Reference. This is a 10-digit number which looks like 12345 67890 and your accountant will help you get it, so get moving !