2002 Honda Civic LX overheating?

I bought this car and the dealer had its garage do inspection. I didn't realize they don't check coolant. So i put coolant and was fine for maybe three days. Tempature Gage was going up to red at stop, then back done when I push on gas. So I checked coolant needed some more, but there NO leak on ground. I have heat in car. And when I open cap in mornings coolant ozes out with pressure. Why? And how do I fix this problem of overheating?

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