I've just begun

I just finished putting under my lights 40 tomato seeds. (I don't know why. I can never use more than 20). Sure enough, the little fungus gnats began coming from nowhere. (I opened a new bag of seed starting mix). So now that there will eventually 3 shelves of seedlings and things germination, how do I control, or better yet get rid of them?

Fungus gnats come when your potting mix is too wet. The easiest thing to do is water from the bottom only, and pour off whatever is standing after 20 minutes. You can also put a light layer of sand on top of the potting mix that will stay drier. Your seedling will come up through it just fine, but fungus gnats don't like it.

Dry conditions is your best defense. Folks tend to overwater more than the seedlings need as a rule anyway.

Once you have them, they're hard to get rid of though. I've been in your shoes, and there's nothing more awful than those little crawly things all over your flats, and room, and t.v. screen, bathtub, piano...

Also, get a small fan and have it blow gently over your pots. That helped me alot.

You can use yellow sticky traps to catch the adults and Bt (mosquito dunks are one form of this) to take care of the larvae. And as melody said, cut back on the watering. Personally I've never had fungus gnats if I wasn't watering too much.