You need to use a Variable. Make a Variable and call it score. Then use the Change score by 1 instruction in your script whenever they score a point - or Change score by -1 if they lose a point.
If you are picking up another sprite you can use a Sensing test and say...
If touching theOtherSprite
change score by 1
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And once you have put the variable on screen, you can right-click on it to change its appearance.
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