When i'm sitting down i notice that my low back starts to hurt also when i get up from sitting i have a low backache. what does that mean ?

Minor strain/spasm: Everyone experiences some aches and pains at some point in life, but if any pain that is adversely affecting your life, you should have it checked out. It is not possible to tell exactly what you have or to better advise you since the information you provided is so limited. Being 24 and no specific trauma, i suspect some low back spasm from prolonges sitting etc... Good luck to you.

Sitting too long: Sitting unsupported creates the most stress on your discs—more than standing, walking. Likely, this simply indicates poor ergonomics. Get up, change positions, reach down and grab your ankles and stretch. Get some aerobic exercise outside of the office too. Core exercises are best: pilates, yoga, and piyo.

Exercise : Need to be more proactive in terms of daily exercise program in terms of flexibility & cardio along with core strengthening while limiting prolonged sitting which generates 4 times body weight across the discs of the lower back. It suggests this is a disc problem . If a smoker stop and if overweight, lose some pounds. You can try otc meds if no medical reasons not to and if persists, see pcp.