"We are not a gang, we are a club!" /nostinkingbadges
Anyway, while you should do some strength/flexibility exercises (start slowly), you can get a lot of benefits just from walking - shoot to work up to at least a half hour and keep picking up the pace, ideally you want to raise your heart rate a bit. Try to do it every day, or almost every day. If it's hard to block out time for it, sneak it into your daily routine - leave 20 minutes earlier and instead of walking to school/work/store directly, walk around the block (neighborhood permitting of course).
This is great advice. Walking, specifically, is what humans are made for.
If you're walking long distances, make sure you buy the right kind of footwear and choose a route which is not hard on your joints. I learnt this the hard way. Thick soles and cushy roads (grass, or some kinds of tarmac) for example are a pleasure to walk on.
A stony path with shoes or thin soles will hurt everything from your ankles/knees to your hips and back. Be careful.
Still it's nice to walk in nature and enjoy the sights.
There's a hill I like to walk on, from which you can see London, all lit up at night. S' Beautiful.
Try mixing in yoga, breathing exercises and things like that.
Meditative practices also have a big effect on metabolism and health. It's worth looking into these things, as they can affect nerves, digestion, pain tolerance etc. through the power of concentration alone. Even if it doesn't make things easier, it gives you more concentration to deal with things around you, and makes you more stable internally too.