The Green House is placed in you agency. The right door in the hall of your agency leads to the Green House.
This option becomes available for your after completing the mission Anger Management (the first mission in Bob Tucker's house).
First of all you have to clean up the Green House and plant flowers. You need to plant flowers only once.
Look after flowers, control weeds (pick weeds out if they grow in your Green House). If 3 or more weeds appear in the Green House flowers wither and you can drop nothing out of them.
Pick flowers and talk to Angela Кramer (click on her and choose in the crafting window trading a package of goods or a flower-bed in exchange for flowers).