NOI basics- experts please help

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ok im sure this has been discussed on here numerous times but I am looking at building a multi family portfolio in the NY/NJ area and Id like to know what Im looking for when I research the NOI and expense numbers on a property... is a 10% NOI on the price of a building a decent return after expenses? After taking the mortgage into consideration and only clearing maybe several hundred dollars a month for myself, does it take a reasonably large portfolio to build wealth in this game? Im young and would like to make a nice living now while setting myself up for a strong future... any suggestions/advice from the players would be appreciated