From the ground up: you'll need sturdy covered footwear with good traction.  Shorts or long pants will usually work on our scheduled trips.  Our guides often wear pants with zip-off legs so they can convert from/to shorts as conditions warrant.  The National Park Service in Hawaii Volcanoes NP mandates long pants for staff working on the lava (as well as gloves) in case of a fall, and they know what they're talking about.  If a customized itinerary takes us off trail in brushy or forested areas, long pants are needed.  A light-weight top works most everywhere, and if you bring along a long-sleeved extra layer, you'll be well prepared.  We supply rainwear and if you prefer to use your own, bring it, too. 

Finally, a hat is a great idea.  We like to wear wide-brimmed hats to give protection from the sun and a dry area around our heads when it's raining.