This one should be learned before high school.
Two has only one use. It’s a number.
Example: I’ll take two of those.
Too is a conjunction meaning and or in addition.
Example: I’d like one of those too.
Example: Too, I’d like one of those. (This use is rare.)
Too is also an adverb meaning excessive.
Example: Those kids are too cute.
To is used in every other case: to form infinitives, as a preposition to indicate place, or to mean roughly for the purpose.
Example: To get to London, to go to the concert, you’ll need to cross the bridge.