don’t count your chickens before they’ve hatched

(proverb) don’t make future plans based on something that has not happened yet

Example Sentences:

Teenager: I’m going to go to Harvard and do an Undergraduate degree. Then I’m going to study Law, and become a successful and rich lawyer. After that, I’m going to go into politics and maybe become the President.
Dad: Okay son, but don’t count your chickens before they’ve hatched. Focus on getting into Harvard first, and then make your decisions.

Similar idioms:

1. jump the gun
2. out for the count
3. believe in
4. have one’s ducks in a row
5. take a rain check
6. come through for someone
7. get through
8. plan B
9. back-up plan