Lemon Juice and baking soda are very effective at whitening your teeth but the acid in the lemon juice can actually erode the protective enamel from your teeth so it's not recommended to use lemon juice and baking soda as a teeth whitener.
A safer way to whiten your teeth is to mix some baking soda and water together to make baking soda toothpaste.
Brushing your teeth with baking soda will whiten your teeth without eroding your teeth enamel like the lemon juice would.