Hi, I would probably do it like this:
Select your player character and look at its properties.
Add a private variable called "score", by default it should be a number and 0.
Add a text box on another layer, name it and set that layer so that it doesn't scroll. This will be what displays your score.
In your event sheet add a new system condition for "Always (every tick)" and then an action: select the text box, then select "set text" - the set text screen will come up as "" don't type anything in there yet, instead double click your player object, then "get private variable" - it will input something like player.Value('Variable name') - replace the 'Variable Name' with what you named your private variable before eg 'score' - keep the quotations. So now, every tick it will set the text box to whatever value the private variable 'score' is.
So all you have to do now is add an action to whatever happens when you hit a coin - so on collision with player and coin add 1 to private variable 'score'