We use 'will' to...
1. Talk about a future event
- Tomorrow, it will be sunny.
- Sarah will be 25 next Sunday.
2. Talk about an action that we think will happen in the future:
- I think you will like our present.
3. Talk about things we just decided to do now:
- You just arrived at the station? I'll come pick you up!
- It's getting late. I'll get going now. (= I'll leave)
4. Ask somebody to do something:
- Will you please turn down the music? (= lower the volume)
(This is a gentler imperative than 'Please turn down the music!')
5. Make offers and promises:
- Here, I'll take your coat. (= I'll hang up the coat)
- You can't come to class today? No problem, I'll send you my notes.
'Will' is a modal verb like 'can'; it does not change in the third person singular (he, she, it).
- I will cook.
- You will cook.
- He/she/it will cook.
- We will cook.
- They will cook.
To negate 'will', put 'not' after 'will': will not / won't:
- I will not be in the office tomorrow.
- They won't stay here.