I am 24 but have a very weak back due to poor sitting & sleeping postures all my life and constantly suffer from backaches. What should I do now?

Physical therapy. Poor posture can certainly lead to low back pain. You might want to consider getting a referral from your primary physician to see a physical therapist to work on core stabilization exercises. Yoga and pilates can be helpful as well in improving your posture as well as strengthening your core muscles.
Exercise. Also, increasing your core strength will help improve your posture.
Exercise. I would love to see you get involved in an extensive core exercise program. This could be started with some formal physical therapy or on your own. You could start with mackenzie exercises you can find through a google search. Also yoga or pilates could be helpful.