Go to the layout view, double click on the screen and create a new Text, call it "Text_Score" for example.
Then go to the event sheet and do a right click, select add new global variable. Call it "Score" for example, select type Number and initial value is 0.
Right click again, select add new event. In the appearing popup window, select System -> Every tick. The event appears in the event layout. Next to it, there's add action. Click on it, select Text_Score, then select set Text. In the appearing small popup window enter the following:
"Your score is: " &Score