You need an accountant.
I’m self-employed as a sole trader so the following is my crude understanding of the situation:
Your tax return is due Jan 31st the year following (so your tax return for the financial year Apr 2012 to Apr 2013 is due by Jan 31st 2014).
You pay your tax in two lumps:
1) Jan 31st you pay half ‘on account’ (for the year coming due, i.e. 2013/14)
2) July 31st you pay second half ‘on account’ (again 2013/14)
Then next Jan (2014) you settle up your ‘on account’ payments for the year 2013/14 when you submit your tax return for that period in addition to paying half on account for the period 2014/15 (and the above pattern repeats).
Don’t forget you need to be VAT registered if your earnings are above the VAT registration threshold.
But I’m not an accountant, so don’t take my word as gospel.