so first of all what do you do with the slime girl essence and the harpie feathers and what not and how do you give the girls gifts I really just need some general info on how to work the game 

The drops from monster girls are used in some quest and you can get a recipe to craft items with them.

You open the menu, select items, select the present item you want to give to a girl, and then select the girl you want to give it to. 

thanks I appreciate  it