I have developed few white patches on my skin recently. i visited a dermatologist and he said it was due to sun damage . suggest me good sunblock?

One that you use: The big issue with sunblocks is people don't use them on a regular basis. Every day when you wake up after your shower you should apply a moisturizing sunblock of 30 or higher. And keep it with you, use it every two hours while you're in the sun or if you sweat or swim. The best sunblock is dark clothes which obviously isn't always practical. Having a hat with you is vital.

Tinea versicolor?: It is a bit unusual to have sun-damaged skin resulting in white-patches at age 18. It is much more common to have a fungal infection called tinea versicolor--this rash typically shows in summer months because everywhere else can tan and these infected areas cannot. The resulting contrast allows you to see the white patches. This is common, but not dangerous. It is treatable. Consult doc. Good luck.