There's no way to guarantee that your budgie, or any other parrot, will learn to "speak." However, budgies certainly have a knack for it, and many times they speak with greater clarity and broader vocabularies than larger parrot species such as macaws and cockatoos.
While their voices are small and gravelly-sounding, budgies have an impressive ability to pick up on human words and phrases, and even using them in proper context sometimes. They are a great choice for those new to bird ownership who want to own a talking parrot.