I am a teenage girl and i think i may have leukemia. what shall i do and how can i be sure?

I was quite ill for a while and i wasn't sure what was wrong, it was sort of like a cold but worse. I always get coughs and runny noses and things and i have recently found a small lump under my arm. I have been researching about leukemia and i think i may have it. How can i be sure and what should i do?