Hi Choco and Welcome to A1
You will need to contact your local tax office to advise them of the change to your status, I believe they will be able to send you some quite helpful leaflets.
Providing you are only using one room as an office, there should be no tax issues (including business rates) with working from home.
Self Assessment tax forms are a doddle. Depending on your circumstances there could be very little to fill in at all. Need to keep a record of any bank interest, share dividends etc for the period. Your first Self Assessment would not be due now until Oct 09 at the earliest and if you were to feel you couldn't complete it yourself, you could employ an accountant to do it for you.
Hope that is some help, look forward to hearing more about your new venture.
All my best
IP2