If you are travelling by land you don't need a visa to enter Canada. You need your passport and your Green card. If you travel by air you need to apply ETA online in their website. There's a fee of $7 and it's valid for 5 years. hope this help
Do I need to know someone in Canada in order to visit Canada
I am a resident of the United States, I would like to know if I need a person to visit Canada for three days
English translation
You might need an eTa depending on your county of origin
You don't need a Canadian visa.
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