Effectively a copywriter is anybody that writes copy or content for any specific advertising or marketing function. It could be creating articles, website content, blogs, ad copy, sponsored social posts, etc. There are a lot of online courses you can take, but it's worth looking at the specific accreditations that each one will afford you. It's also worthwhile looking at other types of creative writing courses so that you can get a intermediate grasp of basic linguistic response, or taking an English Language course. Also, make sure that if you settle on a course that is ratified by an accredited source, not just a 'made up' qualification or a distance learning company trying to sell you distance learning products.