Please speak slowly as I am just now learning Thai.
พูดช้าๆได้มั้ยครับ เพราะผมเพิ่งเรียนภาษาไทยครับ
:D You may put the word "โปรด or กรุณา" at the beginning of the sentence to make it sound very politely. The above sentence itself is still polite, though.
Please note that there are so many ways to say the sentence in Thai.