Chose a product that you are both familiar with and enthusiastic about.
Cost everything before you start. That includes packaging, premises (even if it is just a shed in the garden which must be large enough to store everything and kept secure), trips to the Post Office and postage, returns and suppliers.
Most of all, be prepared for both high and low turnover. No holidays (paid or otherwise)ever again, because you will loose business while you are away, which must be added to the cost of the holiday.