I've looked at
- Sufism -- Sufis -- Sufi Orders
- Project Sufism
- Religion Facts' Sufi page
- Pantheos' Sufism page
- What are the practical differences between Sufi practices versus Sunni and Shiite practices?
but there's so much information I don't know where to begin.
What are the minimum requirements to study Sufism, i.e. what should I know before I start?