Hello Mr .Pathan;
welcome to HealthcareMagic
Since you are 18 yrs now the colour of the skin of face will change because in young age there is tanning
of skin due to excessive exposure to sun while travel;play;driving bike and other out door activities.To make it a little better or fair please try to reduce exposure to sun if it is not necessary to do so.Try applying a sunscreen before going out then after that for care of your facial skin you can get a clean up or even facial done and things will improve.If the problem continues or worsens then contact a Dermatologist
for further advice and treatment.