The Void is a planet you can unlock by obtaining different void keys. You can buy 5 keys for 75 platinum, earn one from logging in, and/or completing missions. Keys have four ranks of difficulty and a preset mission type. there are 4 towers in the void (I believe). You can play tower 1,2,3,4 exterminate, capture, defense, and assassinate. There is a drop down chart on warframes wikia page that show eliglbe missions that might drop a key. Once you have keys you just need to play 10 void missions.
http://warframe.wikia.com/wiki/Void_Keys