How do you get yourself checked for cancer or diseases?

Stories like people who were totally fine being suddenly diagnosed with cancer and such scared me so much…
Sorta gets you to think that it can happen to anyone.

I’m really getting paranoid, especially since I’m feeling some pain sometimes inside. Moreso because I went to a clinic some weeks ago and the doctor told me my liver wasn’t in such a good condition.

HELP?
Oh, btw. I don’t smoke or drink or anything like that (although I do get second hand smoke from my dad occassionally)

I supposedly can’t digest flour products too well (although I didn’t know) and some of my organs are having to waste more energy taking care of that.