You know both will and going to for the future. Use going to for a plan you already decided before now.
A plan you already made: going to
I 'm going to visit my parents this weekend. (I planned it.)
She 's going to start a new job. (She already signed the contract.)
We 're going to have a party on Saturday. (We organized it.)
Try it: Amina bought her plane tickets last week. Her trip to Senfarren is all arranged. Which fits her already-made plan? (She 's going to fly to Senfarren. / She 'll fly to Senfarren.) Answer: She 's going to fly to Senfarren. — She decided and arranged this before now (tickets are bought), so use going to: She's going to fly. "She'll fly" sounds like she is deciding it this second, but she planned it last week.
Something you can see coming: going to
Use going to when you can see it coming now, from what is in front of you.
Look at those clouds! It 's going to rain.
Watch out, that glass 's going to fall!
The bus is full. We 're going to have to wait.
Try it: Hiroshi looks at the dark gray sky. The clouds are getting heavier and the wind is picking up. Which fits what he can see coming? (It 's going to rain. / It 'll rain.) Answer: It 's going to rain. — The evidence is right in front of him (the dark sky, the heavy clouds), so English uses going to: It's going to rain. We save "It'll rain" for a prediction based on opinion or belief, not on what you can see building now.
Deciding right now: will
Use will ('ll) for a choice you make at this very moment, with no plan before now. Also for offers and promises.
The phone is ringing. I 'll answer it.
That sounds fun. I 'll join you!
Those bags look heavy. I 'll help you.
Try it: Mei sees Lars carrying heavy bags. She decides, right at that moment, to help him. Which fits her on-the-spot decision? (I 'll help you. / I 'm going to help you.) Answer: I 'll help you. — She decides at this very moment, with no plan before, so use will: I'll help you. "I'm going to help" would sound like she planned it earlier, but she just decided now.
How to build each one
Two simple shapes. will: will + base verb (I will go, she will help). going to: am/is/are + going to + base verb (I 'm going to go, she 's going to help). The base verb never changes, and you pick am/is/are to match the person.
will + base: They will call you tonight. · I will finish it soon.
be + going to + base: I 'm going to cook dinner. · He 's going to study tonight. · We 're going to leave early.
Match the person: I am, he/she/it is, you/we/they are going to...
Not "I going to cook dinner." — say "I 'm going to cook dinner.". going to always needs be (am/is/are) in front — don't drop it.
Try it: Marta planned this earlier: cook dinner tonight. Build Marta's plan. Put the words in order. Answer: Marta is going to cook dinner tonight. — The shape is be + going to + base: Marta is going to cook dinner tonight. The base verb stays as cook. Never mix the two futures together — "will going to" is not a form.
Saying it will not happen
To make it negative: will → won't (will not). For going to, put not after am/is/are: 'm not / isn't / aren't going to.
won't: I won't be late. · They won't come tonight.
be + not + going to: I 'm not going to buy it. · He isn't going to call. · We aren't going to wait.
Watch out: not goes after am/is/are, not after going: isn't going to (not "is going to not").
Try it: The clouds are gone and the sun is out. It ___ rain now. (isn't going to / isn't going / is going to not) Answer: isn't going to — The evidence changed, so keep going to and make it negative by putting not after is: it isn't going to rain. You still need the full going to (not just "going"), and not comes before going (not "is going to not").
Asking about the future
To ask a question, put the helper first. will: Will + you + base verb?going to: Am/Is/Are + you + going to + base verb?
Will you help me? — Yes, I will. / No, I won't.
Are you going to watch the game? — Yes, I am. / No, I'm not.
Is she going to come? · Will it rain tomorrow?
Try it: ___ you going to visit Senfarren this summer? (Are / Will / Do) Answer: Are — A going to question starts with am/is/are: Are you going to visit...? "Will" works only with a base verb and no going to (Will you visit...?). "Do" is not used for this future.
Which future fits?
Before you choose, ask yourself: Did you plan it before now, or can you see it coming? → going to. Are you deciding this second (or offering or promising)? → will.
Planned before now: We 're going to move house in May.
See it coming now: You 're going to drop that plate!
Deciding this second: OK, I 'll get the door.
Try it: Sofia made a plan last month: she enrolled in a cooking course that starts in June. Sofia smells smoke from the kitchen and decides, right now, to check the oven. Match each one. Which future fits each moment? Answer: Sofia made a plan last month: she enrolled in a cooking course that starts in June. → 1 → She 's going to take a cooking course. · 2 → She 'll check the oven. Sofia smells smoke from the kitchen and decides, right now, to check the oven. → 1 → She 'll take a cooking course. · 2 → She 's going to check the oven. — A plan made last month → going to (she's going to take the course). A decision made right now → will (she'll check the oven). The first was arranged before; the second is on the spot.