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?
There are a lot of options out there for you to explore when launching your new eCommerce site (ranging from utilizing an open source/self-guided platform to hiring an agency to build a completely custom web experience.) But before you think of the "how", make sure and be spend a considerable amount of time around the "whos" and "whats" of your new eCommerce business.
The type of platform, plugins (including dropshipping) and tools you need to be aligned before launching your new store largely depends on the type of product or service you are selling and who are you planning to sell it to. My company has helped launch hundreds of new ecommerce/marketing sites over last 12 years and without fail businesses who strategically and in a detailed fashion map out who their target market is (and calibrate the front end of their eCommerce platform accordingly) as well as think through the nuances of their production/manufacturing/supply cycle (and have their back office tools be congruent with those nuances) are far more likely to succeed. So in order to answer which plugs and which platform/agency i would start with a detailed laying out of all the factors that impact your business specifically and begin to work your way back to start answering some of those questions. With the entrepreneurs/site owners I work with there is a list of questions you can go through and answer as a starting point, but they most resolve around the areas you'd imagine (target market, growth goals, whether or not there is an existing business attached to it, offline sales, digital and content marketing needs, budget/timeline etc.)
Best of luck and feel free to reach out if you'd like to discuss any items in greater detail!