I want to export organic vegetable and thai herbs to singapore.

You should contact the Agri-Food & Veterinary Authority of Singapore (AVA) - since 1.Apr. renamed Singapore Food Agency (SFA).
If your herbs also have (or claim to have) medicinal properties, you need to contact the Health Sciences Authority (HSA) as well.

Your message is not complete. Before asking this kind of question, the foremost criteria is do you have proper certification and your country authorities clearance to export organic products? You should have mentioned clearly about this.

Many countries have their registration process at Government, such as to register with the Ministry of commerce through the Director General of Foreign Trade. Also, register with the Export Promotion Councils which will promote your export goods overseas.

You will also need COO ( certificate of Origin ) and Quality check certificate from authorized agencies. If you have all these certifications and acceptance in place, then drafted out this in a proper email with attachment then send to AVA for their acceptance, as Beppi said above. Good luck