You have one thing TOTALLY WRONG. Sinning does not mean you go to hell, everyone sins, if they say they dont then they just sinned. As long as you ask for forgiveness you will go to heaven.

As for if smoking mj is a sin. The bible says to obey the laws of the land, therefore in the USA and pretty much anywhere besides international waters and the neatherlands, it is a sin. Im not sure about international waters it probably illegal there too.
Humans are, by nature, sinners so dont worry.