Here is the main character of my sci-fi novel:
Cassidy “Cass” Nova
Age: 18
Gender: Female
Background: A trainee pilot on the verge of graduating high school. Born into a family with a long line of pilots, Cass is confident, daring, and passionate about flying. She is the leader of the group, and her dream is to join the prestigious interstellar flight academy.
Appearance: Long, fiery red hair; freckles; sharp green eyes; toned and athletic build.
Character arc: Learning to trust her instincts, balancing her ambition with the needs of her friends, and discovering a hidden strength when faced with tough decisions.
Style: Cass’s everyday outfit consists of a fitted leather jacket with a simple, white t-shirt underneath. She wears dark, ripped skinny jeans with combat boots to give her an edgy, utilitarian look. An aviator’s watch and a few bracelets accessorize her outfit. Her long red hair is usually worn in a high ponytail, ready for action.
Start writing the opening scene of the book, where Cass is partying with her friends on the night before graduation. She sees a lightshow overhead which she first assumes is fireworks, then realises is a space battle up in orbit.

Remember to show, not tell. Keep the pacing slow to increase tension. Include lots of dialogue and description.