You are eligible to enter our weekly raffle for a chance to win as long as you've been active within the app in the past 7 days, and you have not previously won at the given level. Note that winning at any one level does not affect your chances of winning at another level.
To play, you'll need to go to your 'Profile'. Tap on the 'My Health' tab, then the profile icon in the upper right corner. Then tap on the "Play Now" button.
When you see the piñata appear, tap to play and see if you've won.
You may also find the following article helpful, How do I know if I've won?