My advice is to just try and do it! It's much easier to help if you show us some not-working code. Best way to learn is to fail.
But I'll offer some advice. In the room start event you can grab the number of "coins" in the room using instance_number(). Store the returned value in a variable called "total_coins" or something like that. Then have a variable called "current_coins" and set it to 0. And every time the player collides with a coin, increment current_coins by 1 and destroy the coin. Finally, you can draw out these variables to the screen. I recommend using the GUI event for this because all the GUI coordinates are "screen relative". So even as your view/camera moves, the stuff drawn on the gui layer will stay in the same place!
So, give it a shot, and let us know if you run into any problems with actual code. Good luck!