I have no idea about eCommerce and planning to sell my product online. I am not sure which marketing agency to use and which dropshipping is the best?
Amazon. Online stores are dying. They are moving toward boutique status whereas Amazon is what malls or strip-malls once were (before Amazon). If you don't know what you are doing don't try to learn it--lean on Amazon and focus on what you do well (product development, branding, etc.).