Scroll over to the Operators tab on the main menu. Select the "Unlock New Operator" option. From here, you will want to focus on the top tier of operators. These are the "attackers".
There are 5 CTUs and 2 attacking operators for each CTU. That means you have 10 total attackers to choose from. From the SAS, you have either Sledge or Thatcher. The FBI contains Ash and Thermite. GIGN has Twitch and Montagne. From our Spetsnaz friends we have Glaz and Fuze. Finally from GSG-9, we have Blitz and IQ.
You must spend renown to actually acquire one of these attacking operators. If you have purchased a "defending" operator within one of these CTUs, then it will cost more to unlock an "attacker" within that same CTU. Let's say you haven't purchased any operators in the FBI CTU. If you purchase Thermite (which I recommend for people starting the game) then he will cost you 500 renown.
Now let's say you really want to buy Sledge (who I also recommend to get early on in the game). But you purchased a "defending" operator in the SAS already. Sledge will now cost 1000 renown. You see, each operator within a CTU will cost more to unlock. So to purchase all 4 operators in a CTU it will cost you 5000 total renown.
The quickest way to unlock enough renown to purchase one character is to viewing all of the tutorials from the main menu. This will earn you 800 total renown. More than enough to buy one operator.