can you help me, please?

Hi guys!
I'm having trouble.
I don't know when to use the prepositions correctly
When do I use 'on, in, at'?
For example I work 'in' my job or I work 'on' my job"?
Thanks so much! :)

For example:  You work 'in' sales, or you work 'in' the gardening department.  You work 'on' your homework, or 'I am working on the sales report for the gardening department.' Espero que te ayude!