Communication can be an uncomfortable barrier when traveling in a non-English speaking country. Although English is fairly understood, you still want to make sure that the food you order is really what you had in mind. Don't let your lack of Thai language become a hurdle; equip yourself with these basic and very useful phrases. Not only will you get your order right, Thai people will reward your efforts with a big smile and generous portion.
(Bear in mind that Thai is a tonal language, so if you are asked to repeat yourself many times, just point and use a lot of gestures. A smile and friendly attitude goes a long way too!)
mai ped: not spicy
mai sai prik: no chilies
naam: water
naam kaeng: ice
kaao: rice
gaeng: curry, soup
laad kaao: on rice
kaao laad kaeng: curry, soup or other items of your choice on rice
aharn taam sung: made to order food
kaao pad: fried rice
pad kaprao: spicy stir-fried meat with holy basil leaves
kai daao: fried egg (sook: cooked yolk, mai sook: runny yolk)
kai tomm: hard boiled egg
kai jiaow: Thai omelette
tom: boil
yum: spicy salad
pad: stir-fry
todd: fry
pak: vegetables
pla muk: squid
goong: shrimps
nua: meat / beef (nua is also a prefix added to mean meat of something, e.g. nua plaa - fish meat)
plaa: fish
kai: chicken or egg depends on the tone (if it becomes confusing, just point)
moo: pork
guay dtieow: noodles
bah mii: egg noodles (yellow in colour)
sen mii: rice vermicelli
haeng: dry / without soup
naam: with soup
gieow: wonton
look chin: meat ball
ao alai: what do you want? (you might get asked this)
klab baan / sai toong: take away, to take home
gin tii nii: eat here
tammadaa: normal
pi sed: special, extra
tao lai: how much
chek bin: check / bill please
This should be enough to get your message across. Don't hesitate to ask the locals if you're in doubt - there's bound to be someone more than willing to help you out.
Also have a look at: The Four Regional Flavours, Typical Thai Meals & Eating habits, Street Food, Unusual Delicacies, Easy DIY Thai Recipes, An Orientation to Thai Desserts, Thai Fruits and 100 Eating Experiences in Bangkok.