"I have something to tell you" is a phrase that you use when you're announcing something important. Here are some examples of things that you might announce using this phrase:
- You're pregnant.
- You're moving to another city.
- You've quit your job.
- You've broken something that belongs to your friend.
- You're getting married.
- You're getting divorced.
You can also play a trick on a friend by saying "I have something to tell you..." in a dramatic voice, and then announcing something silly like "I ate a sandwich for lunch!"