How can I make my hair blonder?

I have verky long, thick, dark blonde hair that gets tangled easily. As I am still a young teenager, my mum will not let me dye it and is not sure about sun-in. I am currently trying lemon juice and John Frieda lightening spray but I do not really know how to use them in depth. I started the lemon juice thing yesterday and I bought the John Frieda stuff today. The reason I am not sure about sun-in is that I do not know if it will turn my hair orange or brassy. I do not have a lot of time and live in England so sun is limited. Also, I only wash my hair twice a week.