These young budgies have barred forehead, whereas adults have clear yellow or white foreheads by the time they are about 3 months old. Babies also have black eyes, whereas adults will have eyes ringed with yellow by the time they're about 6 months. Buy a budgie that seems active, playful, and bright-eyed. If you want a budgie that may learn to talk, stick with males. People often say females are more prone to nipping, however I find nipping is an individual, rather than sex-linked, trait.
Females make great pets, too.
Most budgies are weaned to a seed-only diet, which is certainly not good for them. Start by adding other foods like fruits, veggies, etc. Budgies are active, playful little birds and do best in as large a cage as you can afford. Most commercial budgie cages are much too small, and the larger ones tend to be fairly expensive. They can cause foot problems.