You need a global variable to hold your score and a text to show the score on the layout.
Make a text and call it score and put the word "score" in it as a place holder. Put in on the layout.
In your events make a gobal variable by right clicking anywhere on the even sheet and click create global variable and call it SCORE.
In your events write this event.
Every Tick: Set Text Score to SCORE
Do not use quotations marks just the global variable name SCORE
That will show the global variable in the text when you run the game.
To change the SCORE variable you can add or subtract a number from the variable.
On Player Collides with Object: Add to SCORE +1
That can be any event you want to keep score of.
Now every time the player collides with an object the variable adds 1 and that is shown in the score text Every Tick.
That is the basics of making a score for your games.