Wait until you start to see the first new leaf growth on your lavender plant this spring. Then you can prune. Only prune last year’s growth back, leaving about an inch of the newer, less woody growth. Harvest lavender flowers just as they are beginning to open, in the morning, as this is when they have the highest oil content and scent.
The University of Vermont Extension has a good guide to growing lavender. https://pss.uvm.edu/ppp/articles/lavender.html