I'm 16, I have recently been buildiing my abs up, but I noticed they are not straight. How can I fix this?

You can't. It's you. Congratulations on your undertaking a fitness program. Nobody's abs are lined up perfectly and you're not going to change this. Many people have a sheet of fascia in the midline, and the tendinous inscriptions are never perfectly aligned and always crooked. Embrace your individuality even as you show off this achievement.