I don't think it sounds stupid; if anything, I think this is a very reasonable question. Tampons can be tricky to get used to, and figuring out where to put them can cause a lot of frustration.
Basically, you have three holes: where you pee, where your period happens (also where babies come out), and where you poop. The hole you pee from is too small for you to accidentally put a tampon/finger into. The hold where you poop is probably pretty obvious to you.
This leaves one hole.
You can find it by gently sliding your finger along from the tip of your vagina crack in a downwards motion. You might notice that you have another set of vaginal lips inside the ones that are showing all the time - your vagina is in here. The hole is not gaping open; it can actually be very tightly closed, but with gentle pushing you CAN get a tampon in.
Make sure you use smaller, teen size tampons and have plenty of time (without the chance of getting interupted!). Read the instructions on the packet, and then try to find your vagina. The tampon does not go in this hole straight up-and-down, but rather you aim it in towards your lower back.
You may have already found the hole, but possibly got the angle wrong.
You can find really useful and clear diagrams online, and your tampon instructions might also include a picture.
Best of luck.