How to be sure you have skin cancer?

I'm young (teen) but I'm pretty sure I have skin cancer and I need to know how to be sure. I won't be able to go to the doctor for a lil bit so please don't be rude. I've never used a tanning bed before but I usually get a tan in the summer without doing much more then enjoying myself outside. I get kinda pale in the winter though so maybe being pale then suddenly getting a tan can do that? Idk, but I never wear sunscreen. SO HOW CAN I BE SURE? Also, I seen something that looks like skin cancer (whole point of the question) just letting you know. I don't think it's melanoma or whatever you call it. My mom says it looks like . . . what do you call it? Eczma I think it is? It's pronounced like ex-eem-ee-ah.