If you are not used to doing it, it is probably easier to do it on paper. Phone your tax office on the 0845 number and ask for the necessary forms.
It will come with an explanatory booklet that explains what goes in each of the boxes.
You will have to complete the form covering your self-employed earnings and any PAYE earnings you did from November onwards - for which presumably you received a P60 at April 08.
HMRC will work out how much tax you owe, which has to be paid before the end of Jan09.