To enter the Shop, you must select the shop icon on the lower right corner of your main screen.
Once you are in, you will find several PowerUps packs (Top Play, Word Spotter, X-Ray Glasses, etc.). You can get these packs with coins. Also, if you scroll down, you will also see different coin packs, which you can get by using real money.
On the upper left corner of the screen, you can see your coin balance and the number of PowerUps you have with their respective icon.