To put things simply: food diversity, lots of veggies, and smaller but more frequent meals! 
Here's a list of essential foods that will help you keep a healthy diet and deliver all the nutrients for your baby's growth!
Fruits and vegetables — raw, steamed, in salads, as much as you can! 
Meat, poultry, fish — a great source of proteins, iron, and vitamin B12 (omit raw food). 
Choose wholewheat products — they offer way more nutrients than their white counterparts.
Dairy products — a source of calcium, proteins, and vitamins (omit blue cheeses and raw milk). 
Keep away from processed foods and the saturated fats in fast foods, ready-to-go meals, ice creams, cakes, and salty snacks like chips.
Drink a lot of fluids (but not alcohol).
Eat smaller portions — it can ease the problem of heartburn common among pregnant women.