WILL

It is used to express a future decision made at the moment of speaking. It is never premeditated.

"Oh! I've let the door open"." I´'ll go and shut it."
"It´s Jane's birthday". "Is it? I'll buy her some flowers"

We often use Will in these situations:

-Offering to do something :  "The bag looks heavy" . "I´ll help you with it"
                                          "Shall I carry your bag?"

- Agreeing to do something:  "Can I have my book back?" "Of course, I´ll give it to you tomorrow"

- Promising to do something:   "I won't tell anyone what happened. I promise. "

- Asking somebody to do something :  "Will you please be quiet?"