It's not an area I have a lot of experience in but from what I have gathered from reading others it's a good idea to start with specific high quality (over quantity) traffic sources which you think match the product best. As narrowly as possible. Unless you have a lot of money to blow through.
This will help you learn what works and what doesn't while minimizing the risk at first. As you get better you can take more risks and branch out to a more broad focus if you want to try that. I'd stick to areas you know well first too and avoid new niches and products at first.
