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So many guys starting out focus on the ones they can see in the mirror. But the real trick is to focus on the ones you can't easily see in the mirror. The entire back, rear delts, and lats. Abs are overrated and worrying about them will probably hold you back.
Obviously you will want to train legs, traps and neck too. I've found the best way of building muscle is training daily, with one day off.
Read this to get an idea, but it's not for beginners