i have a 1998 Pontiac grand am 4cyl. 2.4 l ohc engine and when I turn on the heat the temperature gauge gauge goes almost to the dangerous level.when I turn it off it goes back to normal.what would cause this and how can I fix it?

Most often this problem is caused by low coolant level, or air in the system. Have the coolant system flushed and refilled, warm the engine for a few minutes and recheck the coolant level. Make sure to purge any trapped air by squeezing the radiator hoses, you should feel "solid" water, no air. Your T-stat is in the housing under the water pump, near the firewall. You will have to remove the exhaust manifold in order to get to it.