Address professors using "Dr." or  "Professor." Never use Mr./Mrs./Miss. If you are not sure if they prefer "Dr." or "Professor," sometimes they specify in their course syllabus how they would like to be addressed. Another trick is to search up some of their research online and see how they referred to themselves.

Introduce and sell your skills. Talk about your qualifications and why you think there is a good "fit" between you/your interests and their lab.

Be specific and demonstrate a genuine interest in their line of work.

Keep the email simple and concise - many professors will gloss over the email if it's dense and reads like a book.

Acknowledge your gratitude.

You can include a line stating that you're happy to send them a CV or transcript or anything else they might need if they interested in taking you on.