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?
Start with a website and social media marketing. Website must be SEO friendly and try using WordPress. Add products on the website and then share on social media. Use paid marketing on social media with advance targeting to boost your posts. After you have traffic start re-targeting the customers using custom audience technique.
For drop-shipping compare the price you are getting with the products already available online. Calculate your profit ratio and then decide about it.