You harvest when the cones feel "papery" to the touch. Some wait until the cones just start
to turn brownish in spots.
You can use them wet, but most air dry them to 20% of their wet weight. If you do use them wet you should use them as quickly as possible, but if you dry them you can seal them up and freeze them for later use.
Theoretically you can use them at any point in the brewing process. The conventional wisdom is to use home grown hops for late additions (ie: flavor and aroma), but I used mine for bittering, flavor and aroma additions with great success (I did four batches with all three additions from my home growns and they were favorites in my family).
You could also use them for dry hopping, but I was a bit nervous about using mine for that, in case there were any kind of stray bugs (not literally bugs, but bacteria or yeasts) in them.
Around my area, it's way too early to be harvesting, even if the cones look enticingly large. A lot of the lupulin seems to develop quite late in the growing cycle, so the longer you can keep the cones on the bine, the stronger their payload will be.
As I look back at my log from last year, I started harvesting on August 26. I actually got over 5 pounds dry weight in two harvests from my first year plants last Fall, dried them and froze them, and I used the last of them this Spring.
For drying, I put some wood frames together that held some window screens I "borrowed" from our basement windows. I made three frames, spread my Chinook cones on the first one, then stacked the next one on top with my Cascade cones, then the last frame had my Centennial cones.
I set the stack across a pair of saw horses, positioned a pair of 24" box fans below, resting on a couple of milk crates, surrounded the whole assembly with some scrap plywood panels I had in the garage to channel the air flow through the stack, then turned on the fans.
A couple of days later I weighed the Centennials and they were at that 20% of wet weight mark, so I bagged all the hops and stuck them in a freezer. I got a total of 57 ounces from that first harvest, and the hops were surprisingly potent!