Good question about gear. They do make helmets that are supposedly for mountain biking and they're shaped differently from road helmets, but I don't think there's really any reason that a road helmet would not be okay. Similarly with clothing, mountain bike shorts and jerseys are looser but road bike clothes should still be sufficient. The shoes do matter, since mountain bike shoes have recessed cleats which are easier to walk in. And some cleats are definitely not suited for dirt and mud. Also a Camelback is better than water bottles which would get dirty.

As for getting started, you would want to start with easy trails. I assume there are group rides -- do they do beginner rides?