Plan to: định + verb

Định before a verb means "plan to" or "intend to": Cuối tuần em định đi Vũng Tàu. I'm planning to go to Vũng Tàu this weekend. It's how people talk about plans that aren't fixed yet.

To ask about someone's plans, use it with a question word or with có … không: Anh định làm gì? What are you planning to do? Tết này anh có định về quê không? Are you planning to go back to your hometown for Tết? For "not planning to", put không in front: Em không định đi.

It's also how you say you meant to do something that didn't happen: Em định gọi chị mà quên. I meant to call you, but I forgot.

In Saigon speech, tính is very common in the same place: Tháng sau tụi em tính chuyển nhà. We're planning to move house next month. The longer dự định sounds more formal.

Sẽ says what will happen; định says what you have in mind, and leaves room to change it. Muốn (want) is a wish, not a plan.