"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!"