First, walking is an excellent choice for someone just starting (or restarting) a fitness program. It is something that costs nothing and most people do pretty well instinctively. (If you have pain when you walk, you should see your physician and find out if it is unavoidable or not.) When you walk it is important to keep the arms slightly bent to about 90 degrees bent. Swing the arms counter to the legs and gently open and close the hands. This is especially important for those who notice swelling of the hands during their walks.
Once you have built upto walking regularly, you can add more intentional effort in short bouts with the objective of increasing the effort over time for the entire walk.
The benefits are well documented and as said above, a simple search of “benefits of walking” will give you millions of sites. So I am not going to bother with that here. If you come to be considering trying to increase the intensity or challenge of your walks, I strongly urge you to speak with a fitness professional. Your entire structural support system can be forced out of alignment and underlying issues could be aggravated un-necessarily by improper progression. Adding weight for instance, requires preparation of the muscles that will be required to maintain posture and support the weight. And never add ankle weights, these are almost always a bad idea no matter how fit the individual.