I'm a math student at the second year of college and I'm approaching category theory for the first time for personal interest. An older student suggested me to start by reading "Categories for the working mathematician" by Saunders Mac Lane. One exercise recites as follows: "Describe coproducts (and show that they really exist) in Cat, in Mon, in Grph. My only idea is to construct the first with a disjoint union of the objects of two given categories and do the same for the arrows however I'm not sure that is allowed with classes. Any suggestions for the other two execises would also be helpful.
ps:I'm sorry for my bad english (it's not my mother language)