1. "В общем" means "in general", "on the whole" or "all in all" (as a parenthetic word)
В общем, ты всего лишь женщина, детка = All in all you are but a woman, baby.

2. Sort of "as a result", "finally" (referring to a reason containing in the previous sentence)
В общем, он стал хорошим человеком = As a result, he became a good person.
"В итоге" works in these cases, too.

3. "In short" (as a parenthetic word)
В общем, это сложно объяснить = In short, it's hard to explain.
Also you may use "короче говоря", "словом".

You see it at the beginning of sentences in 99% of cases.

But sometimes it's used differently.
Они смотрят на проблему в общем = They look at the problem in general. (they don't consider nuances, they need a whole picture)

Also be careful and don't mix up "в общем" as an adverb with "[в] общем" as an adjective in the prepositional case. Because "общий" means "common" or "general".
Это его доля в общем имуществе = That's his portion in the common property.
"Общем" is an adjective in here.