Best thing to do is to start with small regular exercise and work up from there. I've jumped into trying to be more active than my body was ready for before and you just feel rough if you do that which puts you off doing it again. Try and find something you enjoy or some other motivating factor like music or through other people - eg classes or being active with family or friends.

If your starting goal is weight loss then the number one thing to do is to look at diet and then some kind of regular cardio exercise. The healthcare service here says that adults should aim to do 2 and a half hours of cardio a week. Also, especially in terms of metabolism, you want to eat less than you need but not by a drastic amount as your metabolism can slow as a response which doesn't help you out. Keep in mind how your body will get energy as you exercise - the more demanding the exercise the more your body needs energy quickly so will break down more carbohydrates which are faster to breakdown than fats which are slower. More fat is burned at a slower pace.

Strength based exercise that is aimed at muscle building will burn calories too and by increasing muscle you increase metabolism (although it is a small amount).

I personally started off doing an hour of cardio a week which was a dance-based fitness class. I then did another couple of hours of dance fitness at home a week and started to do strength based exercises too - probably a couple of hours a week. I have an active job so have to eat a lot to make up for it all but I naturally have quite a high metabolism.