Think back to what you did yesterday. It doesn't have to be sport. You can count a brisk walk to the shops, gardening, vigorous housework, dancing... anything that made you breathe more heavily than usual.
Your aim is to build up to 30 minutes moderate activity a day.
If you can't manage 30 minutes a day, anything more than what you're doing now is a step in the right direction.
If you can't manage 30 minutes in one go, two 15-minute sessions are just as good.
If you're doing 30 minutes a day already, keep it up - and do more if you want to.
Remember - anything is better than nothing!