The south rim, the most popular side with visitors, including hikers, lies at about 7000 feet elevation. It is open all year round. The Colorado River, which has carved the canyon, lies about a vertical mile below. Some five million people visit the south rim every year. Very few ever see the bottom of the canyon. Although many do start down the Bright Angel Trail, they soon realize that they should not go very far. Even then, some get overextended.
Likely, you will find that an overnight hike into the canyon will be more enjoyable than the especially strenuous one-day hike in and out. This will take planning and preparation. You have the choice of taking the Bright Angel Trail, about a 10-mile hike from the south rim to Phantom Ranch, or the more steep South Kaibab Trail, just to the east, about a seven-mile hike to Phantom Ranch. Phantom Ranch is a comfortable destination for a two-day trip. You can spend the night there, where cabins and meals are available, or at a nearby campground, where tent sites are available.
Reservations for Phantom Ranch must be made well in advance, typically a full year ahead of time. Also, there is a very small window open for getting permits for the limited number of camping spaces at the Bright Angel campground. The permits are snapped up very quickly. (Similarly, reservations for the mule rides into the canyon require a long lead time. Each mule, incidentally, carries no more than 190 pounds.)
After you establish the preferred dates for making your hike, find out when the National Park Service will accept applications for those days. Get your application in as early as possible. This guarantees a date that your permit will arrive. You will have to follow all instructions precisely, getting the latest up-to-date information. Also, provide alternate dates in case you don't receive your preferred date. Remember to hike safely and take time to enjoy your surroundings.