While this seems like it may be the easiest question to answer, it is essential to give it a lot of thought. Finding items that are readily accessible is key. Not every item will be a thousand dollar unicorn.
There is an absolute learning curve that can be flattened a bit learning your niche before ever listing your first item. If you can find a market that peaks your interest, and has a good return of investment, it will be much easier to get moving in a positive direction.