Depends entirely on the wind. Some have none, most have some, and many have squally winds that may overpower you if you don't shorten sail.
The wisest course is to reef early, and make sure you have enough searoom to leeward of the anticipated wind direction. Don't count on being able to make any distance to windward during the worst of it. Just reach, or close reach, and luff as needed. Put on lifejackets too (and a safety harness if you have one). Pay attention to your compass, as wind direction may change a lot during a squall, and you won't be able to see squat during the heavy rain. If the wind's just too much, I'd say drop all sail, anchor (assuming now you're not too deep to anchor), and wait it out. Usually in 20 minutes the weather's good again (not always, though).
The other concern is lightning. you can't do anything about it, but stay away from the mast and stays, they're electricity conductors. If you have a cabin, your crew is safest down there.
I can't be much more specific, as I don't know whether you're sailing a cruiser or an open boat, and not all t'storms are the same either.