When do police have to read the Miranda warning?
If police arrest you without reading the Miranda warning, that's normal. Any statements you make after your arrest are admissible if they are not responses to questions.
If police arrest you AND they start asking you questions, they must read you the Miranda warning. If they don't, anything you say can be thrown out. So if they start asking questions, don't say anything! The only time it is ever appropriate to answer their questions is when you have an attorney with you.