If you aren't already a walker, start slowly and build up over time! If you try to jump in and go for long walks right away, you'll wear yourself out, and you may get frustrated and give up. Be patient with yourself!
To begin, you might go out for 10 minutes, up and down your block, for a few days. Then add another few minutes to your routine, until you're up to 30 minutes a day. Or walk three times a day, for 10 minutes each time. If you are very sore or tired the next day, back off a little and go for a shorter walk.
Especially if you walk outside during the winter, make sure you dress for the weather. It's discouraging to get outside and be too cold to finish your walk, or worse yet have to finish your walk while cold because you're not near home! If you dress in layers, you can always remove or unzip a layer if you get too warm.
For many people, the hardest part of any exercise program is sticking with it. Make a plan for yourself when you'll walk each day, and don't let other distractions, whims, or responsibilities take that time away from you. Walking with a friend is a great idea, because you're also accountable to someone else.