Raise vehicle by using a floor jack on the side jack points. Use of jack stands is highly recommended.

Remove the vehicle's wheel being extremely careful not to put any part of your body underneath the car.

Visually inspect the tire for any damage including nails, metal fragments, dents, tears, etc.

If no damage is found visually, there may be an issue too small to see.

In a clean, empty spray bottle, mix dish detergent (not hand soap) with water and shake well.

Generously spray the tire with the soapy water. Ensure you spray any possible trouble areas like the inside edge of the rim, tire tread, etc.

Inspect the tire for leaks such as this one. When an air leak is present, the soapy water will cause large bubbles to appear from the air trying to push it's way out of the tire.

Pinpoint the issue that could be causing your tire to leak. Issues can range from a simple nail in the treads to a crack in the rim, like the wheel in this photo.

Now that the leak has been found, deal with it accordingly. Nails in the tire tread and hairline cracks in the rim can be safely fixed, depending on the location of the damage. Use only reputable, trained mechanics to fix wheel issues.