Day hiking in Colorado is an exhilarating activity. Few things can compare with standing on top of a mountain. The system of trails in and around Denver is top notch - well maintained and well traveled. Generally, a short day hike is considered an easy outing, but there are still some things you should do before heading out.
Learn the basics of hiking. There are lots of books available, some online references, and classes at outdoor retailers. These will provide general information about the most comfortable clothing and shoes, and other gear that will make the trip more comfortable.
Learn about your destination. Takes maps with you. The JeffCo site is a good place to start.
Pack water and snacks.
Watch the weather.
Begin early, to beat the crowds and the summer rain.