It all starts with value investing. To invest wisely for the long-term, one must understand the underlying economics of a business.
You must enjoy accounting.... yes we said it.
If you do not, you will always be at a disadvantage again those who do, and like it or not, investing is a competitive pursuit. You need an edge anywhere you can get it.
There are a great many pure value investing sites on the web, but a good place to get started is Wikipedia.