The first time you have access to the Citadel, you can find Diana Allers at the Normandy Dock: Bay D24.
Choice: Invite to the Normandy or not
When you speak to her, she’ll make a proposition to join your Normandy Crew as a Journalist. You’ll have the two choices: “Let’s do a trial run” or “No way”.
Choice: Let’s do a trial run
By agreeing to her proposition, you’ll find her on the third floor, in the Starboard Cargo Bay, on the Normandy. At major plot points throughout the game, you can have an interview with her, depending on your answers, you’ll get increased Military Strength of the War Assets related to the topic she’s interviewing about. The type of reporting she’ll do is determined by you choosing either the rally support or expose Cerberus options. These will change what missions will get interviews.
Choice: No way
By not agreeing to her proposition, she’ll end up on another ship, the Alliance dreadnought, SSV Shasta. Later on, in the Citadel, you can overhear the news reporting that the SSV Shasta was destroyed in the Horse Head Nebula, resulting in the death of the entire crew, including Diana.