Coming out of your house to a flat tyre is something we all dread and very often it will happen for no apparent reason and with very little warning. If the tyre has developed a slow puncture you may have had to fill it with air more often that you should have but it may have all of a sudden got worse.
The first thing to do is to pump the tyre back up, ideally this should be in situ as if you drive on a totally flat tyre it is likely you will not only damage the tyre but also even the wheel trim or alloy.
Once pumped up inspect the tyre for any visual damage such as nails. If you do not find anything you can chose to spray a solution made from one part soap to four parts water all around the rim and over the surface of the tyre to see if you can see any bubbles coming up from where the air is escaping. If not then you will probably need to take it to a garage for a further inspection.