Apart from restaurants, you will find food almost anywhere in Bangkok: inside markets, along the pavement or anywhere a cart and a few fold-up tables and chairs can be placed. These offer cheap and tasty food you won't find in normal tourist restaurants.
A few basic rules of thumb apply: if the food looks fresh, the oil in the pan isn't dark sludgy brown and the food is thoroughly cooked in front of you, it's generally safe to eat. Stir-frying, grilling and soup made from boiling water offer safer cooking methods. Avoid eating anything raw and avoid crushed ice which might have come from a dubious source. Follow these basic rules and go for it!