This isn't really AI unless you are going to get into fuzzy logic to decide what the characters are. Anyhow, the steps are:
- Image capture.
- Identify the number plate - look at shape recognition algorithms for this.
- Extracting the registration number. The key word here is Optical Character Recognition [OCR] and there are existing libraries for this; ask Dr Google.
If you can write your own OCR routine - irrespective of if it's for number plates or not - you'll be on line for top marks as it's frogging hard. Good luck.