Been self employed for 5 years now. Best way is to get an accountant...that way you don't have to deal with tax people (they are scary!!).
I'm the UK so don't know if this applies over in different countries. I save ALL my receipts! Literally everything....make-up, hair, nails, phone bill, electric bill, my coffee...
You get an allowance of £11k in the year for your earnings (in the UK). £1 over your taxed 20% on that (so 20p to go to the tax people). So you earn £12,000 you will be taxed £200.00 on the £1000 you are over however if you can show you have spent £200 that is to do with your business in receipts you pay no tax or very little...
Your accountant will tell you what you can and can't claim ...I put in all my Asda (Walmart if in USA) receipts in and they got chucked out LOL...but my dog food didn't!!
XX