To vs Too
Quick answer: To points somewhere or starts a verb (to school, to run). Too means 'also' or 'excessively' (me too, too loud). The extra O in too is the extra meaning.
To
Preposition/infinitive marker: toward; part of a verb form.
“We walked to the library.”
How to remember it
Too has one O too many — use it when there's 'extra': also / excess.
Common mistake
'I want to come to' — the second one means 'also': too.