Here is my website on various methods of NFP.
The sympto-thermal method is perhaps the most effective, but also difficult to learn. The Standard Days Method is relatively effective, at about 95%, but it is the easiest to learn. The SDM is a modern update of the old calendar (or rhythm) method. SDM can be learned merely be reading about the method online, although also taking a class would be beneficial.
There is a website comparing the Standard Days Method to other natural methods: