Well, to get Nekros, you need to do Orokin Derelict Assassination (ODA) mission (not Orokin Void). Now, to get to ODA, you need to buy ODA key from the market and craft it.
To craft to ODA key, you will need Golem Nav Coordinates.
Golem Nav Coordinates drop in every Orokin Derelict mission (Capture, Defense, Survival, Extermination, Assassination)
To get to Orokin Derelict missions you will need to buy Orokin Derelict Capture/Defense/... key from the market.
To craft these keys you will need to get Nav Coordinates. These regular Nav Coordinates drop in every mission in the game.
EDIT: or you can just take a ride with someone