I did what your asking for in my game.
Create a global variable for the players wallet/inventory/totalMoney etc..
Create an event that add's 1 to the variable everytime a player collects a coin.
Create a variable for each power-up have it set to "0" if the player has not purchased the powerup.
Create a check that looks to see if the player has enough coins, and hasnt already purchased the power up.
If player has coins to purchase, set upgradeONE.instanceVariable(purchased) = 1
Have your power ups coded in so that once the player has purchased the upgrade, the upgrade applies to them.
Create a game asset for your power up(art) and a store art asset (a pop-up window sprite etc..).
When player wants to purchase a upgrade from your shop. Provide them away to open the store (a button to click on..)
Here is a link to a "How Do I" section, very useful