Travel alone or in a group.?

I was wondering if its better to travel alone or with a travel group?I'm 18 and never travel before but i know my street smarts.I would like some websites for travel groups with tours or websites on how a new single traveller should do.I don't know where to start but i want to plan out something for the summer.I'm tired of never been out of canada.please post sites or information thanks.:)