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.