You'll have no problem using only English in Thailand, but trying out a bit of Thai will endear you with the locals. Here are some useful basic phrases to get started.

• Hello! — Sawasdee(Add krub if you are male, kha if you are female)
• How are you? — Sa Bai Dee Mai (krub/kha)
• Good morning! — Aroon Sawas
• I am fine/I am good — Chan Sa Bai Dee
• Thank you — Khob Khun (krub/kha)
• How much? — Tao Rai (krub/kha)
• Discount please! — Lod noi dai mai (krub/kha)
• How many? —Mee Kee Chin (krub/kha)
• Delicious — Aroi Mak Mak
• Spicy — Ped
• Too spicy — Ped Gurn Pai
• What time is it? — Kee Mong Leaw
• I am tired — Chan Nueai
• Where is the restroom? — Hong Nam Yu Nai
• Sorry/excuse me — Kor Thod (krub/kha)
• The bill, please — Gep Ngoen (krub/kha)
• Can you take my picture? — Tai Roop Hai Noi Dai Mai (krub/kha)
• I want to go to the supermarket — Chan yak pai supermarket
• Happy to meet you! — Dee Jai Tee Jer Gan
• Goodbye! — Sawasdee (krub/kha)