There are many different ways in which fish can act and many things which can effect their behavior.
The most popular things to look out for are:
If your fish are spending a lot of time at the surface, this could be due to there being a lack of oxygen in the water therefore they are gasping. A blocked filter or lack of plants or too heavily stocked is the most common cause.
If you find your fish are not well, closed up, lethargic and not eating then the most common cause for this is the water temperature. Closely monitor the temperature of your tank and make sure your heater and thermometer are working correctly.