Rüdiger's answer is correct. 'Hallo' can as well be used for people with whom you use formal German, such as your work colleagues, a shop you frequently go to buy some stuff, but usually not with people you meet for the first time. There are of course regional differences, but following this advice you can avoid some plunders ;)

"Guten Tag" is more polite and formal which means it is a good option when meeting new people. It is in the accusative case! This because it is an abbreviation of "Ich wünsche dir/Ihnen einen guten Tag"

People Say Guten tag is offical and polite way, Guten Tag+A, Tag is der, so gut+en. hallo say to friends or children.