In French, when you get overly excited about something, there are many expressions such as “Ne vends pas la peau de l’ours avant de l’avoir tué“.
 
In English, it literally translates to “Don’t sell the skin of the bear before you have killed it”.
 
On the other hand, a common phrase in English would be “Don’t count your chickens (before they’re hatched)”, literally “Ne comptez pas vos poulets (avant leur éclosion)”.
 
This expression means to be more careful and to not get too ahead of one’s self.
 

If someone you know is starting a business and tells you they are going to make so much money before even starting, you could say “Great! Sounds exciting, but don’t count your chickens before they hatch.” to inform them to be more cautious. They will thank you later.