How can I straighten my back spine without surgery?

Probably not. Bracing for scoliosis during adolescence is controversial. Even the supporters of it generally think it prevents worsening, but it does not make the curve go away. If the curve is progressing, consider surgery, especially if it is affecting your appearance and self image. If it is not progressing, stay physically and socially active.
Depends. If you have a true, structural deformity of the spine, only surgery can correct that. Many mild deformities certainly don't warrant that. If there is a non structural component to the problem, this can be treated non operatively based upon the problem. For example physical therapy for poor poetical muscle balance or a shoe lift for leg length discrepancy, etc. Thank you for the question.
Depends on reason. For your abnormal posture. If it is a spine deformity at your age, it will not change without surgery. If it is poor posture or related to leg muscle contractures or tightness, then flexiblity and core conditioning exercises will help. If it is due to a leg length discrepancy, than an orthotic to correct some of the difference may help.