Hi TirzahLaughs,
Both can be correct, but here's the catch: you must pick one and stick with it throughout.
First, you have to decide if your narrator is telling the reader a story that has already passed (this is usually a framed story, where the narrator makes it clear at the beginning that they are going to tell you about something that happened, e.g. "It all started on my tenth birthday...") or if the reader is walking alongside the narrator through the story. Both are valid narrative structures. In the first case, use past tense. In the second case, use the present tense. And never vacillate in between.
To use the example from this thread:
"So, I slapped him silly when I found out and he got mad and jumped in his car. I'm flipping him the bird while he's driving away staring..."
The issue is "slapped" is past tense, while "I'm (am) flipping" is present tense.
It could be:
"So, I slap him silly when I find out and he gets mad and jumps in his car. I'm flipping him the bird while he's driving away staring..."
-or-
"So, I slapped him silly when I found out and he got mad and jumped in his car. I flipped him the bird while he drove away staring..."
Good luck untangling!