I have a canon lens FD 100-300/5.6 which is not compatible to my camera (canon 600D). I wonder to know does it worth if I convert the lens to my camera? (make it compatible to mount on).
Converting the lens mount to EF so it can be used on an EOS camera would be very expensive and not worth it. Even if it were an "L" lens it would be difficult to justify the expense.
Using an FD to EF adapter will produce poor image quality due to the additional optics required to maintain infinity focus.
Your best bet is to buy a mirrorless camera because all Canon FD lenses can be mounted using a simple glass free adapter.