Dear Dr. []:
My name is [] and I am currently a [state what you currently doing] at []. I admire your research on [X] and I want to ask you about the possibility of working with you towards my [masters/PhD]. [Insert a few ideas or questions you have about their research on topics you might want to work on]. Are you accepting graduate students in the next year?
Thank you for your consideration and I hope to hear from you soon,
Sincerely,
[Your name]
[Your current affiliation]
[Your contact email]