It’s more than being a good writer
If you want to be an online copywriter, it's not enough just to be a good writer. Good writing helps, as does good grammar, but ultimately, one looks to a copywriter to engage and sell.
You have to be able to keep the reader engaged, but you also have to be persuasive and drive the reader to action. If the reader doesn't take the desired action, i.e., buy the product you are promoting, then it doesn't matter how well you write, you have not converted your traffic.
If being an online copywriter is what you really want to do, spend some time reading promotional sales material and see if you find it persuasive. When you find some that would encourage you to buy, dissect the article, and see what their tactics are. That's what you want to mirror.