This piece for Malachain collects the advice that holds up in practice.
Walking is the easiest form of exercise to start and keep, because it needs no equipment, membership or special skill. To make it count as moderate activity, walk fast enough that you can still talk but would struggle to sing. Start with ten or fifteen minutes at that pace and add a few minutes each week until thirty minutes feels comfortable.
Variety keeps it interesting and builds fitness further. Include a hill or a flight of steps, try short stretches of faster walking between lampposts, or choose a new route at weekends. Comfortable shoes with good support prevent most aches. Walking with a friend or a local group makes it far more likely that the habit will survive bad weather and busy weeks.
There is more to learn on this subject, and the linked website goes into greater depth. Treat this article as an introduction rather than the final word, and check any important decision against up-to-date, reliable sources before relying on it.
If you try any of these ideas, start small and adjust them to your own situation. What works for one person will not suit everyone, and it is fine to keep only the parts that help. Writing down what you changed, and what happened, makes it much easier to see after a few weeks whether the effort was worthwhile.
The most useful step is usually the first one. Pick a single suggestion, try it for a couple of weeks, and then decide whether it is worth keeping. Small, steady improvements last longer than ambitious plans that are hard to maintain once everyday life gets busy again.