Write a good business plan! Planning is by far one of the most important things to consider. Plan out your sales strategy and how you will get yoru product/service out to the market. Plan out your financials and what types of loans or funding you will need to get started along with how many sales you will need to pay all of that back. A well written, well thought out business plan will encompass these things as well as require you to do research on your competition and further flush out the positioning of your business.
I am not saying that every single decision you make needs to be well thought out and planned. Starting a business is all about taking chances and risks, but the more informed you are, the less risky your decisions may be.