You will need to produce a set of accounts (income and expenditure).
Any costs that are incurred wholly and exclusively in your business will be tax deductible.
However, the usual expenses would be:
Printing, Postage, Stationery, Mobile phone costs, Business calls (not line rental) on your home phone line, Accountancy fees.
You can also claim capital allowances if you use your car for business (business proportion only), and on computer costs, furniture etc.
You are then taxalbe on your net profits, as adjusted for tax.
Without knowing more about your business I can't give any more specific information.
If you are unsure about what you need to prepare accounts etc., I would suggest that you get professional advice for at least the first year accounts and tax return.
www.taxingtimes.ie