Here are the rules to the puzzle as said by Wikipedia
"In the missionaries and cannibals problem, three missionaries and three cannibals must cross a river using a boat which can carry at most two people, under the constraint that, for both banks, if there are missionaries present on the bank, they cannot be outnumbered by cannibals (if they were, the cannibals would eat the missionaries.) The boat cannot cross the river by itself with no people on board."
If you want to solve it in a more interactive manor you can play the game
If you get stuck here is my step by step instructions using a gif made in Photoshop (click it to see it animated):
