I don't recommend fasting for just weight loss. I don't know how an 18 year old can gain so much weight. Also do t know when is your meeting with a person you are fond of. You can expect 1lb per day weight loss.
Where to start? Just stop eating. You may start with just juicing, freshly squeezed organic fruits and vegetable juices, consuming right away after pressing.
No vitamins.
Filtered or distilled water is fine. Drink warm water.
No brushing teeth with toothpaste, just use water, scrape your tongue with tongue scraper to keep the tongue clean.
No chewing gum or mouth washes. No exercises, walking is ok.