Offer a direct and honest expectation. Make clear both your expectations for salary (your monthly cash compensation) and total compensation (salary, bonus, and any non-cash compensation). Additionally, make clear your compensation growth expectations. Ask about benefits in addition to compensation and be ready to adjust your required compensation if benefits are less than the norm.
I'm expecting $_____ in total compensation, with at least $_____ as salary, and annual increases of __% assuming our work is going well. Let me know if this is similar to the compensation planned for this position.
Your tone in the overall conversation has more to do with how your response will be received than the specific wording. Additionally, you may find it easier to be polite and friendly in person or over the phone so consider moving your conversation off of email.
It's likely that you and your future employer will agree on compensation without negotiation - be sure to propose a level of compensation you would be comfortable with.
Avoid any negotiation strategies that would cause you to make requests you wouldn't otherwise bring up in the discussion.
If you're unsure of what typical compensation is for your role you could use online databases or reach out to friends/colleagues in similar roles. You should also check with any undergraduate or graduate institutions you have a connection to - many operate databases with just this kind of information for alumni.