How can I treat a hot water burn scar?

Red burn scars can be treated with pulsed dye laser.  In my practice, I often use a combination of IIT and fractional laser for white old burn scars. 

Without seeing the scar it si difficult to provide you with recommendations. We can never eleiminate a scar but we can make it less noticeable, conceal it along normal anatomical landmarks, soften,. flatten, lighten or change the direction.

RE - "I got burnt by hot water when I was young. I got surgery and it left me with a scar. What can I do to remove it? " It sounds like you had a full thickness (Third degree) burn which required removal. Such burns are usually treated by replacing the burned, dead skin (eschar) with a skin graft.

Unfortunately, no surgery or device (laser etc) can erase a scar. In some cases, we can make the scars look better and more acceptable but complete removal / erasure of the scar is still an impossibility.