The basis of your pet parakeet's diet should be a dry seed mix formulated specifically for parakeets. Sprouting seeds, fresh fruit and veg, a cuttlefish bone, and a mineral block complete the menu.
If you only offer dry seed to your birds for several months when you first get them, they may 'lose the taste' for any other type of food. This is something to avoid, as a bird that eats only dry seed may become overweight, especially of the seed mix contains lots of oily varieties such as millet.

Go easy on the millet!
Parakeet Food in the Wild
In their natural habitat in Australia, budgerigars eat mainly grass seeds. These are eaten at all stages of the grasses' development - when dry and ready to fall, freshly sprouted, or taking root. Parakeets also eat leaves from their favorite nesting and roosting tree, the eucalyptus. They will nibble fruits if these are available, and get themselves labelled as pests by descending on fields of barley, wheat and other crops.
Poor diet is responsible for the majority of parakeet deaths, something that may seem surprising if you think you've been supplying nothing but the best, in the form of a quality dry seed mix. The problem is that older seed loses its nutrients. Malnutrition can therefore be the result. The same problem may occur if the parakeets eat too many nutritionally poor but filling treats, such as bread, pasta and rice.
Parakeet Water
The birds' water should be changed every second day. Even bowl protected from falling food and poo will accumulate feathers, dust and seed husks as the parakeets flap around. They sometimes use their drinking bowl as a bath, too. The water needs to be clean – if it's fine for human consumption, it’s okay for parakeets too.

A water dispenser attached to the cage minimises spillage
Parakeets' drinking water does not need to be fortified with vitamins or minerals – that will all be provided from the healthy diet. If medicine needs to be added to the water, clean the bowls and drinkers thoroughly every day.
Simply, 7 June 2022
I disagree with this. Budgies do like seed, but it should be offered as a treat, not part of their main diet. If it is part of their main diet there is a high chance of fatty liver and other diseases. Instead, offer vegetables and pellets. Avoid brands like ZuPreem or Kaytee, though. Tops, Harrisons, and Roudybush are great pellet brands. I use Roudybush, and it costs the same if not less than seed if you buy a 44 oz off of amazon.