I agree with what CellyCell said. You might want to be wary if your hair isn't already naturally light in tone, otherwise it would either not lighten enough or the results would end up too brassy.
As an alternative you can try Sun-In. (results vary from person to person) But my mother used this on my younger sister's hair, which is dark brown but looks black at times, but is very reflection. It gave her nice, natural looking brown highlights. (But I don't think she even used a lot, my sister's hair naturally lightens by the sun itself quite easily)
In addition, I don't think even Sun-In or lemon juice will drastically change dark hair or lighten as an all over color. I think if you were wanting to achieve sun-kissed highlights that would be a much better option.
ps: I would never advise anyone to sit with lemon juice/Sun-In in the sun for too long. Since it's acidic it can actually promote photosensitivity. And that's never good! And there's also that risk of drying out your precious locks! I would recommend going to the salon and getting professional coloring instead.