You don't need to tell us their backstories.
But you need to know their backstories. Each character comes from somewhere, went to school somewhere - or worked, or survived in some sort of wilderness, or survived in some sort of slum or escaped prison. They had a loving family - or an absent family - or an overly strict mother - or an uncle who exploited them for labor... a domineering and competitive older sister... Maybe they came to Star Fleet after first serving in a local military on their home planet.
We'll get tantalizing bits of those backstories eventually - someone will mention, "you learned that from your older brother?" - "I don't want to talk about it." or "I'm going to show you a little trick I picked up in a Cardassian prison..."
Those backstories will tell you how each character thinks, how they behave, how they react, how they talk. Don't worry about not having presented much of their personalities yet. Their personalities will emerge as you write them.
Build the environment. Set the scene. Present a problem. Call your characters to the stage. Then just follow them around, watch what they do and how they react... and describe that in detail to your readers.
Thanks!! rbs