You must have British nationality to be eligible for a British passport.
An adult passport is valid for 10 years.
Apply for a first adult passport if you’re aged 16 or over (or will be within 3 weeks) and never had a child passport.
You can use your child passport until it expires, even if you’re over 18.
Follow the process for getting a first adult passport if the last passport you had was an old black or blue passport.