When is the best time of year (northern hemisphere) for me to plant a herb garden?

MUCH too vague. You have to find out what zone you live in, and that is just the first step; here are some sample zone maps of N. America, Mexico, Hawaii, Australia…where’s home?

gardening zones

The easiest way to start is to walk around neighborhood, see what – if anything – is in bloom, talk to your neighbors over the fence and stop in at the local gardening center or nursery. Gardening in Anchorage is quite different from planting in Acapulco.

What Gail said. The first step is to figure out what zone you live in. If you’re going to be planting seeds, a lot of times the packet will have a zone map on the back and a chart listing the correct sowing times for each zone.

As a tip, if you’re using seeds, you might want to try starting them indoors. You should be able to buy little seedling containers at a nursery or a home improvement place that sells plants. They’re fairly convenient because once you’re ready, all you have to do is pop the soil and seedling out of the container and then transplant.

Sowing indoors will prevent birds from eating your seed (seed-eaters have an instinct to scratch on plots of upturned dirt) as well as keeping any snails or bugs from eating your seedlings. Then you are free to transplant them outside once you deem them strong enough. This also lets you plant several seeds at once, which is helpful because not all seeds are viable, so you may not get a plant from each seed you sow. If you end up with several plants, you can pick out the strongest/healthiest of the bunch and plant those.

Definitely check the zone you are in. Here’s a handy website to figure out where you are; http://www.usna.usda.gov/Hardzone/ushzmap.html
Of course, this assumes that you are on the North American Continent. Also, I would suggest plangting in planters…some herbs are very invasive.