Separate bank accounts and deductions

Hi,

I am just starting my Sole Proprietor crochet business officially this month (registered with NY state and have an EIN).  Last year I was able to deduct some of my start up expenses on my regular tax form.  I am looking for information about how business expenses and deductions work.  NYS recommends a separate bank account to remit sales tax.  They also recommend an account for income.  Then here on TurboTax it warns against using a business account for expenses because they aren't all deductible?  So do I just keep purchasing yarn and similar supplies with my personal credit card and just claim it in the end?  My goal had been to get the business self sustainable where the income would cover the costs of the business and anything left over was my profit.  But now I don't understand what to do and I don't want to do anything wrong and ruin my financial life.

-Daria