We can use the auxiliary verb WILL to express future.

FORM: Will + verb

affirmative I will be a doctor = I'll be a doctor.
negative I will not be a doctor = I won't be a doctor 
interrogative Will you be the next president?

The negative contraction won't is pronounced with a diphthong, like "phone". Don't confuse it with the verb "want", that is pronounced with a vowel, like "got".

USE

We don't use WILL to talk about personal plans. For personal plans we use "be going to" or the present continuous.
- On Sunday I'm going to visit my grandmother.
- They can't go with you to the concert, they're coming with me to a party.
We use WILL to talk about a decision we make at the moment of speaking
- Josh, I'm going to the doctor
- Oh, I'll go with you, wait.

We use WILL to talk about predictions.
- I think the Real Madrid will win the Spanish football league.
- I'm sure that Mark will be the next president.
- This stick is very strong, it won't break.

We also use WILL to make offers and promises
- I will help you if you need it.
- I will love you forever.

WILL or BE GOING TO?

Very often these two forms are interchangeable, but in general, and simplifying things, we can say that we use "be going to" to talk about plans and "will" to talk about predictions. For the rest of things people often use both forms. WILL is more formal and more common in writing, and BE GOING TO is more informal and more common when speaking.

- Do you know if he will come tonight? = Do you know if he's coming tonight? = Do you know if he's going to come tonight? = Do you know if he gonna come tonight?