Simple Future: Will and Be going to

(a) Jack will finish his work tomorrow.
(b) Jack is going to finish his work tomorrow.
(c) Anna will not be here tomorrow.
(d) Ann won't be here tomorrow.
Will or be going to is used to express future time*. In speech, going to is often pronounced "gonna".
In (d): The contracted form of will + not is won't.
*The use of shall with I or we to express future time is possible but uncommon in American English. Shall is used more frequently in British English than in American English.