Welcome to the first stage of the game, this stage is Super Bell Hill (1-1). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
You will find the first green star near the third Horizontal Pipe. Go through the first entrance and quickly tap or tilt Up to grab the green star.
Green Star 2
After reaching the Checkpoint Flag, look for the rabbit standing to the right of a tree. Capture the rabbit to claim your second green star. You should go find the stamp right after getting this star.
Green Star 3
Climb the mountain after green star #2. Go after the giant rabbit and get the Mega Mushroom. While being supersized, run to the right of the bridge to break some rocks. The last green star will be found inside the hidden room.
The stamp can be found inside the green pipe under the Giant rabbit mountain.
Other Notable Secrets
Right under the bridge where you had gone supersized. Go under the bridge to receive a secret 1 Up.