Sorry about wall of text. TLDR go once a year + research your health insurance now when there's not an emergency + medical directives and medical power of attorney and plan ahead stuff + check into what stuff your insurance covers yearly.
Basically, go to your general doctor once a year. "Family practice" doctors work on everyone, children and adults. "Internal medicine" doctors are for adults. That's the basic distinction, but you can go to either. Most of the time there's not really any reason to stop going to the doctor you've always gone to.
Learn about your insurance, what's covered, what's not covered, and it's very very important to know who and where is "in network". Never go out of network unless it's a life or death matter.
Also, may sound morbid but you're an adult now and it's not too soon to have a medical power of attorney and a will drawn up, just in case.
They say guys don't need them before age 50, but it wouldn't be out of the question to get a colonoscopy (and a look down the throat too, forget the name) around age 30 for guys.
If you poop blood and it's bright red, that's probably not much to worry about. If you poop very dark black coffee grounds then that's internal bleeding and you gotta go soon.
Had a friend die of colon cancer age 39, I had a scare age 30, lots of polyps, now I go every 3 years.
Eye stuff: helpful hint with insurance -- in USA, generally if you don't have eye exam insurance, you can go to an opthamologist and they will write you a prescription plus take a look in your eyes. critical if you have diabetes. Optometrist is the one at Target and Wal mart who write you a prescription for eyeglasses. Opthamologist is the one who can do surgery on your eyes. That's the one your medical insurance will pay for.
If you are generally healthy and not overweight, then you should still probably go once a year for a general all over wellness exam if your insurance pays for it. You could make some notes over the year of any concerns. Especially when you're young, the vast majority of the time it will be a bug or something that goes away in a few days.
Read up about appendicitis, pancreatitits, gallstones, kidney stones so you can at least have that in the back of your mind. Those would possibly be the things a healthy person would have an issue with. Don't freak out about WebMD saying every fucking ache and pain could be cancer! cancer! cancer! No it's not cancer.
If you snore bad, you could go in for a sleep study.
If your eyes get yellow then you gotta go to the doctor, that's jaundice and your liver. Most of the time it's not life threatening but it's also not minor.
Don't mix tylenol with alcohol. If you get put on antibiotics, no alcohol until you're through with the antibiotics. Always take the entire run of antibiotics (and eat yogurt or you'll get the runs) even if you feel better.
Get your flu shot, look into hepatitis shots. Probably should get STD tests every 6 months if you are sexually active. It's not that uncommon to have no symptoms.
If there's a fever over 102 or so, get to the doctor or urgent care (not the ER).
Other than that, it's probably not anything to worry about.
You're young so here's a very important insurance tip: Always make sure wherever you go is "in network" for your insurance plan. For example, my insurance covers PrimaCare clinics but not CareNow clinics. They're half a mile from each other.
If you ever have surgery, try to make sure the entire hospital is "in network". Make sure that every single person who interacts with you is "in network". Especially the anesthesiologist -- they are more expensive than your surgeon. I just go to Baylor, where everyone and everything is in my insurance plan.
Do the insurance investigation now, save that info in your phone so you'll know where to go if you cut yourself really bad.
Source: my sick, disabled, middle aged self.