Can anyone advise if there is a good website/program that shows how to write good content to improve a websites performance.
Here in the United States, they have this classes available in the High Schools, things like "English" and "English Literature". They can be kind of boring for sure, but it's amazing - you may actually use them at some point later in life. No program can show you how to write good content. You learn the basics, then you practice, practice, practice. Writing ... like typing, is an acquired skill. Not everyone is good at writing.
I think the best way to write good content is to really think about what your reader wants and start writing from there. I don't know of a program that does what you're asking for, but I spent a year working with a professional writer, and I found that to be very helpful. Stephen King's book, On Writing is an excellent resource for how to write well for your audience. As for search engines, there's lots of information out there on how to do that too.