It depends on your skin and what you are lightening. Basically darker pigments in skin are caused by greater amounts of melanin. If you are young and not deficient in Glutamine, you don’t need to take more. To lighten patches of hyper-pigmentation and melasma, I would tend to use Kojic acid, Liquorice extract or A-Arbutin (a natural derivative of hydroquinone) or even Niacinamide. These inhibit the production of melanin (they do other things too) so the darker bits will fade. You can also try Vitamic C (Ascorbic acid) or even Lactic or Glycolic acid, if your skin is strong and doesn’t have a lot of blemishes or broken skin - these will help speed up the process as they encourage the top layer of skin cells to shed, leaving the new, less pigmented skin to come through.
Then, yes, wear a good, broad-spectrum SPF (it is important to get broad-spectrum so that the UVA lights are blocked, not just the UVB). If you are trying to generally lighten your skin, not just patches of abnormal colour, that is a different matter. You can only really stop/slow tyrosinase from activating melanin and the materials listed above all do that to a greater or lesser degree. If your skin is darker than you’d like it, due to melanin being activated by sunlight, then you can, with some concerted effort, go back to the colour you were. Choose whichever you can source easily (using two or even three together can speed up the process), but remember that some can be quite harsh and using them together could make your skin sore. Some only work in a certain pH, for instance, Vitamin C needs a low pH) . Then just stay out of the sun or under a good sun-protector. You should only need to reapply the treatment if you have been in sunshine hot enough to tan you, it shouldn’t return on its own - although the melanin is produced deep down in your skin, so it may take a while for all the skin layers above it to move up and slough off; you only see the top layer.
Be careful trying to lighten your skin to a colour lighter than this. If that is what you want, it would be worth speaking to a dermatologist before trying anything that could damage your skin. The lighteners I mentioned will all take some time to work, depending on how much melanin has been produced, and how long it has been there and where, in your skin layers it is. It could take between 2 and 6 months to see a real difference and longer still if you want to reduce it completely. Most of these will only fade it, not completely bleach the skin and some skin responds better than others, so expect to see the dark patch fade 50–60% and you may get a bit more, if you stick with it. Don’t try lemon juice, as this can give you serious burns if the skin is exposed to daylight, even on a cloudy day.