If you haven't exercised in a while, a walk is a good place to start.
Here are a few tips to get the most out of your walk:
Be aware of your posture. Keep your back straight, but not stiff.
Let your arms swing freely.
Choose shoes with good arch support.
Set a schedule and stick to it as much as you can.
Plan your route and you can eventually change it if it no longer motivates you after a while.
FYI - I LOVE walking. My legs are so much stronger for all the walking I do.
Start. Walking. Today.