Greetings! Thanks for query.
Darkening of your face is due to tanning from UVB and UVA rays in the sun light. So the most important step is to prevent further tanning by regular use of sun screen offering protection from both rays. You need to apply sunscreen 30 minutes before sun exposure and reapply every 3-4 hours if you are working out doors.
There are many skin lightening treatment available in the market. I would suggest you to see a Dermatologist for suggestion of right product based on your skin type.
As the saying goes "prevention is better than cure" so make it a habit to use sunscreen on regular basis.