I stuck a tampon in my vagina on my very first period ever, and it wasn't even a slim-fit sized one!
It's way, WAY easier to do it while on your period. Of course you have a vagina, if you didn't, where is the blood coming from?
You really should experiment in the bathtub some night and feel around gently to find your entrance. It's pretty important to know where it is! You WON'T see an opening even with a mirror.
Insert your fingers first, when you're not on your period. Find out where your vaginal entrance his, how big it can get, and how deep you can go it.
Go buy SlimFits tampons, they're skinnier than normal ones. They're more like the size of one finger.
Trust me, if I could stick a full-sized tampon up there without ever having put anything up there on my first period, you can do it now!