A recommendation for monads. Don't read too much tutorials, try it. Programm in some languages that emphasises monads and then reflect how you used them. I think these fundamental concepts are way easier to experience than to explain.

Edit: what are toposes and why do you want to learn it? Has it some connection to topology?

For monads, learn the following concepts in order:

Category

Functor

Natural transformation

Adjoint functor

That's it.

If you don't understand what monads are you should probably wait with topos theory. It quite abstract and you should probably be comfortable with both algebraic geometry and category theory before you approach it