I have dry brown spots under my armpits and theyre spreading up my arm and down my sides. So I have cancer? Is it an infection? What do I do?

See a doctor. This forum is great for general questions, but this sounds like something you should go see your doctor and/or a dermatologist for.
Fungus? It could be superficial fungus infection because of color and it's spread. This is easily treble once diagnosis is confirmed with external antifungals. See doctor for proper diagnosis. The description is not suggestive of skin cancer.