An ambassador is a person who represents a company or organization. They are usually chosen because they have some connection to the brand, and they are considered to be influential within their community.
To become an ambassador for a brand, you typically need to have some sort of relationship with the company or organization. This could mean that you are a loyal customer, or perhaps you have worked with the brand in some capacity in the past. It's also important to be active on social media and have a large following. If you meet all of these criteria, you can reach out to brands that you're interested in working with and inquire about becoming an ambassador.