Okay, so Americas route.
spend time with him during breaks like with other characters
On the first day during break he will ask about how are you feeling at this school. Answer "Honestly, you're the coolest person here."
During English test always choose second option. If you did everything right, he will ask you to come over and 'study'. Say "Sure".
At his house ask "Do you like anime?", "Nyan Neko Sugar Girls", "Kawaii Metal is cool, eh?" (You are able to give gift here)
During Math project join America, Russia and Ukraine. Then choose:
"Please calm down, we've got to work on the project, right?"
"Calm down, please"
"Let's use blue"
During break, at schoolyard, stay with America.
In class, when you sit with America, choose to talk to him.
After that, during break, at schoolyard, say "Alright...".
When he asks where Canada could be, choose third option.
During break, at schoolyard, choose "You should tell Canada yourself".
Once Canada and America made up, you can visit America during break at schoolyard and give him a gift.
Basically after they made up, there won't be any choices there. Just hang out with him during breaks and in the end write him a love letter.
Hope that helped!