Start your copywriting journey...
1. There's a difference between a copywriter and a content writer (although they're often now seen as the same thing). A copywriter persuades someone to do something (eg. buy a product). A content writer writes articles to fill space and/or build reputations and company profiles. Think of it this way "a copywriter sells, a content writer fills".
2. Writing good copy is no different from writing good content in one major way - if it's not interesting, people won't read it. And if they don't read it, they won't buy.
3. Becoming a great copywriter takes a lifetime. This is the mindset you must start with. If you're not willing to invest your life in learning to become a great copywriter, do something else. By 'lifetime' I mean some people become proficient in a year, some take 20 years, some never (because they give up).
4. To become a great copywriter you need to write every day (every day you don't write is a day lost you could have become a better writer). Read every day (read from a critical perspective). Read everything (books, newspapers, magazines, articles, online content). There is no one good source of copywriting know how (it would be impossible to contain it all).
5. The only test of a good piece of copy is the sales it makes. Once a piece of copy has some sales stats, it becomes the control piece all further tests can be measured against. This is how you win new customers (by showing them results) and retain existing ones.
6. The opinion of other people about your copy is just that, an opinion. The only real opinion comes from a happy buyer of the product your copy sold.
If you take copywriting seriously, it will change your life and the lives of many others along the way.
International Copywriters Association
PS. If you'd like help becoming a professional copywriter, join the ICA.