Start with your needs. Do you need a subscription service, or just the ability for people to pay one-off? Do you need fast and easy, or robust and integrated into an uncommon business accounting software? Is the product digital download or a physical product you're shipping out of a warehouse/store?
I've worked with over a dozen of the leading ecommerce payment services, and will walk you through the best options if you set up a call where we discuss your needs.