I'm not sure what you mean...
There a basic instruction
Following the steps:
1-You can create a global variable it would be called "Score" in event sheet.
2-Create a Text Object
3-Make an event "Every Tick", make an action for Text, choose "Set Text" it appears an Expression window, go to that and open click, you see list of variables, double click Score or type Score WITHOUT quotes " "
4-Do whatever when you destroy an enemy, it should make a new action "Add to variable", select Score and create value 100.