Guide books are essential. The best ones are recommended at appropriate points in chamonix.net. Aclimbing instructor is a great way to learn or tackle challenging routes. It can get hot. Guide books describe the orientation of the crag so plan the climb when it is in the shade. Don't forget to take water.
The lifts open for the summer around June 15th but the Midi lift operates year round, except for scheduled maintenance usually in November.