Go on day hikes first, then overnight hikes.Before you set out for a multi-day trip, try a couple of day hikes through various kinds of terrain and weather to find out how it suits you. It's a good idea to make sure you enjoy hoofing around out in the woods before you find yourself out in the middle of a 14 mile loop in the wilderness.
Pick a general destination for your backpacking trip.Are you interested in the mountains? The grasslands? The great lakes? Depending on where you live, the backcountry may be close by, or you may want to venture out farther for a serious hiking experience. In most areas, you shouldn't need to travel more than a half a day by car to find a good National or State Park in which you can hike and camp.
Pick a specific park or wilderness area.Want to hike the Cumberland Gap? Explore Yosemite? Pitch a tent in the Grand Tetons? Once you've settled on a particular region of the country you'd like to explore, pick an area that's appropriate for backcountry camping. Within the US, here are some of the best destinations for serious camping: