Jake the Snake isn't all that long. You'll need to get him to grow before he'll be able to reach the exit on each level. Even then it isn't always easy to figure out how to get him to the exit without Jake falling into the water or hitting dangerous obects.
Have Jake eat all of the stars on a level to enable the exit. As Jake eats the stars he will grow. Don't let Jake hit any of the dangers on the levels or fall into the water. Keep in mind that the stars can act as a platform and can support Jake.
Use the WASD or arrow keys to control Jake or on touch devices swipe with your finger.
If you want to reset the levels (re-locks all the levels except level 1) use the Clear Data button on the Levels menu.