There’s a lot of writing advice on the web, and it can be hard to find out which advice is best for your situation. If you’re looking for a place to start, I would recommend the following from Copyblogger on writing with power and authority.
I would also recommend studying their archives. There’s a lot of free, actional content you can use to build your own content library, and you can use all of this material regardless of how successful you are right now, or regardless of how you measure success. All it takes is a serious time investment on your part, but if you’re trying to run a business and make sales, you already knew that you have to invest a lot of time and hard work.