The general rule is not to pick off more than 1/3 of the green. But you do want to pick and use your herbs. Picking encourages them to branch out and grow much fuller and bushier. But let them get a decent growth going first. You don’t want to pick too much too early. Like I said, no more than 1/3 of the green.
Once you get your sage going, you won’t be able to kill it. It’ll last about 3 years. (I find that after that, the taste somehow alters, and becomes more bitter.)
Mint will invade everything in your garden once you let it loose. Be careful of mint. It propogates by spreading runners through the soil, so once it takes root, it’ll be everywhere unless you take good care.
Basil is my favorite. It will want lots of sun, water, and well-draining soil. Don’t drown it, but keep it from getting too thirsty. This is the plant that benefits the most from religious picking. By the end of the summer my four basil plants grew to three feet high by about three feet wide. Lots of pesto last year! And don’t let it flower!
Also, do not use fertilizer for herbs. They taste better by being a little bit deprived of nutrients.
Some other herbs you might want to consider for your gardern:
Lavender–lovely smell, and you can use the flowers or green for herbes de provence
Chervil–a very delicate green herb, similar to parsley with an anise-like taste to it; one of the French herbes fines. prefers part shade in my experience.
Dill–make some homemade pickles this summer and use your fresh dill!
Summer savory–the “bean herb”, also good mixed in tomato-based sauces with basil, cilantro, and mint
Cilantro–although it’s so cheap at the supermarket here, it’s not worth growing
Mint, on the other hand, is virulent enough that it would be considered a pest were it not fragrant, so if you manage to kill it, I’ll be very impressed!