What exactly to say on it so it looks professional?
As little as possible. Mine usually say, "I am resigning my position effective (insert date)." Signature.
A resignation letter is not the place to slam the employer, vent about why you're leaving, or give details about where you are going (especially if its a competitor). Let silence rule after your letter is delivered. Perform your work to the best of your ability until your departure date and leave your current employer with grace. (Never burn your bridges even if you feel like dumping gasoline on them and throwing a match.)
Whatever is customary in your country?
Even though we have people from all over the world on these boards, most of us here are in North America (Canada and USA), and you need to take the advice with a grain of salt.