The most important factor in caring for your tires is maintaining the correct tire pressure. It is difficult to tell, simply by looking at the tire, whether or not it is properly inflated. The weather can have an important effect on tire pressure as well - you need to add air when it's colder outside and let air out when it warms up in the summer. Also, be sure to check your tires after your car has been sitting for a while. When you drive, the friction from driving heats up the air inside your tires and increases the tire pressure, which can give you an inaccurate reading.
Tires that are underinflated wear out faster and create excessive heat. In addition, underinflated tires make your engine work harder and can lower your fuel economy by 0.3% for every 1 psi the tires are low. Put another way, driving with properly inflated tires can improve your gas mileage by 3.3%. We recommend keeping a portable air compressor in your trunk so you can top off your tires at home - they are inexpensive and plug right into the 12 volt socket in your dashboard.
If one tire is lower by more than a couple of psi than the others, it is likely you have a small leak (from a nail or other object). Getting this fixed is likely to be inexpensive and can help prevent a more costly and dangerous blowout.
Fun fact: Around 2 billion gallons of gas could be saved each year if all Americans kept their tires properly inflated.