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