Starting again my 9 miles challenge I had to quit cycling a couple of years ago and now I am back. I started cycling to university 9 miles one way which took me 1.30 hour. I know it's longer than what it should be, but I've just started.

I want your advice please: what food should I look for healthy energy?  I can't walk now, my knee kills me ... is that normal?

Any other advice?
9 miles should not require any changes to your diet, excluding extreme temperatures or excessively mountainous terrain. Your knee hurting is likewise not normal. Your bike may be a poor fit, or you may need to learn to use a lower gear: spinning the pedals faster, with less force required on each stroke.

Neither fatigue nor knee pain should be an expected outcome of a short, low-intensity cycling trip, particularly for someone of typical university age. If either of these persist, I would strongly recommend consulting with a physician.