Do your research would be my number one tip. No point having the best looking website in the world if the products you list are available elsewhere for less. Make sure you understand how things like google ad words work. Again if your website doesn't come up in searches nobody will know you are there. Make sure you are familiar with online trading laws (distance selling, consumer rights etc) in the area you are going to sell to.
Most of all be wary of scams. You will be bombarded with spam with people offering to help your business. Good luck.