One reason the Qur'an can be difficult to understand is that its chapters are generally arranged by size (longest chapters toward the front, shortest toward the back), rather than in chronological order.
This site presents the chapters chronologically, in a time-sequence accepted by scholars at the world's chief center of Islamic learning, Al-Azhar in Cairo.
Early Meccan Surahs
Middle Meccan Surahs (618-620)
Late Meccan Surahs (620-622)
The 28 Medinah Surahs (620-622)