most chain auto parts stores have code grabbers and will read your dtc
(diagnostic trouble code) for free.
most likely it's a simple deal like an O2 sensor but the code will tell the
tale.

 Hi
 I bought a second hand Mitsubishi Eclipse and it shows a message "service
 engine soon".

 Can anybody tell me, what might be wrong with it.

 I suspect that you've crossed the 60,000 or 90,000 mile mark or whatever is programmed for a major service. Depending on how much you've got invested you may want to do the service that was intended with the warning or at least get things checked out.

 Bob Walker