99 Chevy Malibu Overheating PLZ HELP

Hi.. I purchasae a 99 Chevy Malibu a few days ago. I took it in to my uncles shop yesterday and had the ac charged and bought 3 new tires... Today I was driving and the check coolant lite came on and then the gage shot up to all the way hot.... The car started to steam under the hood. I waited a bit for it to cool and then noticed that the lid was not all the way on the coolant reservoir... My uncle said this might actually be a good thing... I waited for it to cool a little more and went to the gas station and put in a gallon of water and made sure the lid was on really tight... and drove thirty miles to my uncles shop. Mind you on the way to his shop the gage did not go over the halfway mark.... So i get there and thy looked for signs of antifreeze on the engine etc and did not see any.. They said that becuase the cap is presurized if not put on correctly it would have possible caused the car to overheat,, So about an hour ago i went to get my son from school and it did go up two llines above the half way mark on the temp gage on the dash? Is that ok? Any ideas? I saved up the money for this car,,, am a single mom,,, college degree and cant find a job.... PLZ HELP ME>>> Thanks