This depends on the type of trail you are being taken on. If you have any say in where you go, insist on an amateur trail with few major hills and with easy terrain. Make sure your guide/hiking buddy knows that you are an amateur so that they can give you additional instructions. If they don't know that you're new to this, they might assume that you know things that you don't.
Also, make sure whoever you are going with is an experienced outdoorsman/woman. If no one in the group is very experienced, stay on clearly marked amateur trails, bring a map, and don't go into densely wooded areas. If a group of rookie hikers wander into an extremely remote area, that's just asking to end up on the 9'o'clock news.
Don't freak out. Hiking is fun and healthy recreation, it just requires a steady head, and some common sense and experience.
As for getting in shape, walking the treadmill with incline (if your treadmill has that option) and stairs are a great way to practice for hiking. Just remember, that with a backpack and boots, actual hiking may be a lot different than your indoor treadmill experience.