The Association of Avian Veterinarians recommends the following diet for a parakeet:
50% cooked beans, whole wheat bread, cooked rice, pasta, and seed 45% fresh broccoli, carrots, yams, spinach, dandelion greens, other green/orange fruits and veggies 5% eggs, tuna packed in water, well cooked meat
Parakeets are lactose-intolerant and cannot eat a lot of dairy product. Also, parakeets are HIGHLY allergic to chocolate and avocados and should never be given either. Junk food such as pretzles, potato chips, etc. aren't good for a parakeet either! You can feed a parakeet fresh fruit such as apples and cherries, but be careful about the seeds as those have toxins in them. Lettuce is fine, but it really has no nutrients in it. It's better to give them other, healthier greens.
Parakeets do NOT overeat. If anything, people tend to give them too little food, thinking a seed cup is full when really it's just full of the hulls of the seeds the parakeet has already eaten. Be sure to refill your parakeet's food supply daily and to give him or her lots of fresh foods too.