Hi there,
As stated by Domo, you need a good accountant to help you get tax relief on everything that you're entitled to. I am self-employed and have since my 1st payday put 25% in a separate bank a/c for the tax man so I wouldn't be tempted to spend it
My expenses are usually quite high so I've found that 25% is sufficient so far. Given the changes in todays budget it may be better to put away 27-30% and have some left over rather than a big shock.
Next year - August/September, take you paperwork to an accountant and they'll prepare your tax return. You will be expected to pay the tax for money earned in 2010 and a pro-rata amount on account for 2011 earnings.
Keep
every receipt even for little things like biros, highlighters etc.
Best of luck in your new venture