Hi and welcome megadeals4u2010,
The answer is RESEARCH. By the way, where are you planning to sell? You can start by choosing a market, then start your research.
If this will be your first time to sell, I would suggest you try selling a few things from around your house. More like a test sale so you can get your feet wet and get the feel of things. Starting out this way has its advantages - no capital needed, meaning you also have zero to little risk factor; you get to practice your selling skills (taking photos, making listings, writing your product descriptions); good way to build up your seller feedback plus you might discover a niche while your at it.
You might also consider a hobby as your guide in choosing the products you will sell. As they say it is easier to do and succeed when you are doing something you are passionate about.
There are many similar posts in the forum, do find time to read through them. And don't forget our Education section, I'm sure you'll find the guides and articles most helpful!
To help you with your advertising needs, check out this post - Link hidden: Login to view
Hope this helps! All the best :)