To tell your mother about this, you could say "Mom, I have something to tell you." The phrase "to tell someone about something" is a common way to share information or news with someone in Engpsh. Here are some example sentences:

"I have to tell you about the party this weekend."
"I need to tell you something important."
"I want to tell you about my new job."

In each of these sentences, the speaker is letting the pstener know that they have information to share, and that they want to tell the pstener about it. The specific information will be shared in the following sentence or conversation.