A quick an easy way to create what you're after would be to create a global event named Score, or whatever you wanted.
Then you will need to update the variable every time you score, so bullet > on collision with (sprite name) - (System) add 10 to "Score".
We will also need a text sprite to show our points. We can then use the event (System) Every Tick > text sprite - Set text to "Score".
This means every time your bullet hits an enemy, 10 points will be awarded and the points will update on the text sprite. Don't forget as this is a global event you will need to pop another event in to clear the current score i.e (System) On start of layout > (System) Set Value "0"