Affiliate Marketing Secrets

Hi,
I will be traveling to the U.S from Canada. To do a 4 month internship with a reputable company. However, I will not be recieving school credit, or pay for my participation. Im confused as to what visa, if any I need to obtain.
Ive been told by some I dont need any since Im not affiliated with a school, nor will i need a green card because im not getting paid. And since its under 6 months I should be ok to fly over, do the internship and return without further documentation.
Can someone provide me with some clarification or knowledge on what i should do? or if you know the visa type… than that would be even more helpful .
Thank you.

I have to move to Korea due to family problems, but would I pass admissions with…
-ok grades… A’s n high B’s
-US citizenship
-Father with US citizenship. Mother with green card.
-9th grade going to 10th
-Played viola for 2 yrs.Piano for 7 yrs.
-Lived in the US since 6 months
-born in Seoul Korea
-figure skate 3rd in state. Also golf team in 8th grade basketball team 7th
-4.31 GPA
-parents are not affiliated with the Korean gov or a Korean business (Samsung, lg, Kia, etc)
-upper middle class. Enough to pay for int schools
Sorry for the grammar I was planning to go to YISS cause it’s near where my Korean apartment is.
Thanks