There are 5 CTUs and 4 operators in each CTU. It will cost 5000 Renown to unlock all Operators within any given CTU. So you are looking at 25000 total required Renown to unlock every Operator.
You earn renown by completing the Situations and playing Terrorist Hunt and Multiplayer. You can also earn a quick 800 renown by watching the Tutorial videos on the Main Menu screen.
Here is a breakdown of the CTUs and the operators in them:
Attackers: Sledge, Thatcher
Defenders: Mute, Smoke
Attackers: Ash, Thermite
Defenders: Castle, Pulse
Attackers: Twitch, Montagne
Defenders: Doc, Rook
Attackers: Glaz, Fuze
Defenders: Kapkan, Tachanka
Attackers: Blitz, IQ
Defenders: Jager, Bandit
Now, everyone has their own unique play style and probably can play better with different operators but I suggest to even out your purchasing of operators. Don't just buy all attackers and then focus on defenders. That would be stupid. By one or two attackers at a time and then even it out and buy one or two defenders.
My two recommended attackers would be Thermite and Sledge for new players. For defenders, try Kapkan and Rook. Those 4 are very easy to get used to and can make this game go a little smoother for you. Operators like Glaz and Smoke are a bit more for seasoned players.
A small warning, Twitch is fairly useless especially if you have a Thatcher on your team. I tried using Twitch and found out quickly that there are much better attacker operators. If you want to be a team player, choose the operators I recommended up above. If you are a lone wolf style of player, then Ash and Glaz would be your attacking operators. For defenders, Pulse and Tachanka are great for flying solo.