Greetings! You have worked out correctly that skin tanning is due to sun exposure. Sun light has harmful rays (UVB & UVA) that cause darkening of skin and premature ageing. The most important first step is to prevent any further damage by avoiding sun exposure esp. during peak hours (10 Am to 4 PM) and taking precautions like wearing wide brim hat if you cannot avoid going out during this time. The second step is using sun sunscreens. There are different brands of sunscreens available in the market. To be effective it should be at least SPF 30 or more and should be applied 30 minutes before sun exposure. The effect of sunscreen lasts only for 2-3 hours so you have to reapply it after this time. You also need to keep in mind that no sunscreen will give you 100% protection. Now coming to treatment for tanning, there are many products available that lightens the skin tone. Please see a dermatologist who will suggest the correct treatment after assessing your skin. But if you are looking for natural ways of skin lightening I would suggest you to search on the internet and I am sure you will find lot of information on this subject.