Fear of Cancer! Should i do now?!

Hello doctors, I am 27 years old. I am 5".4' and I weigh 73 kg. I don't understand how much my weight should be according to my age, gender and height. What kind of nutrition I should take and additionally how much exercise I should be doing! I am extremely afraid that once I start exercising and I lose weight, that might  mean I have cancer. I am diagnosed with Lichen Planus in my legs (pics attached are Lichen Planus spots). I don't know if this is in my head or is this something that should not be the way to think or live! I keep on googling about Cancer, various forms of it and think to get myself tested about the same. I am extremely confused about the whole thing. I need help, can someone please make me understand how weight loss is related to cancer and how much weight loss is actually unhealthy?! Additionally, should I start doing some exercises or what exactly should I do now?! P.S - Whoever responds, thank you so much for understanding my position.