Welcome to the site! It's great to have you on board.
Generally it's best to start selling something you are interested in because you are going to be spending a LOT of time thinking about it! Do you love fashion or movies? Perhaps you feel excited about the thought of new gadgets and technology? It's likely that there are plenty of other people who also share your interest - giving you an instant audience for your product.
Anyway, here are a couple of ways to find out what is selling well on eBay:
1) Check out the popular searches and most popular products on eBay:
Popular searches: Link hidden: Login to view
Best selling products: Link hidden: Login to view
eBay pulse: Link hidden: Login to view
2) Perform completed listings searches for products or brand names to see the conversion rate and average selling price. To do a completed listings search, simply click the Advanced Search button next to the
main eBay search box. Enter your search keywords as usual, and then check the Completed Listings box. Hit search and you'll see historical data for the previous 30 days. Successful items are in green; unsuccessful items are in red.
Hope that helps!