It’s a great idea to get to know your body! Sometimes if the vaginal opening is barely visible, there could be a thin layer of tissue (called the hymen) covering the hole. All girls have a hymen, but some cover the vaginal opening more than others. Occasionally, the hymen could cause difficulty with tampon insertion or even menstrual flow, depending on how much of the hole it is covered. Make an appointment to see your healthcare provider and talk about your concerns. If he or she determines that your opening is covered too much, you can discuss options for treatment.
A barely visible hole is perfectly fine. I think the female anatomy is incredibly amazing with all the adapting it can do. An area that doesn’t appear to be large enough for its intended purpose will end up having no problem doing what needs to be done. If you are really interested in learning about your anatomy, I would suggest having a diagram or two nearby while you are exploring. Don’t rely on just one; the human body is far too diverse to be represented well. A few different options will make you feel more comfortable with you have compared to what is represented.
Props to you for getting out a mirror and checking yourself out! I’m not an anatomy expert, so it’s hard to answer your question with certainty. I do know that sometimes it can be hard to see everything down there when you’re trying to get the mirror at just the right angle, so I think it’s possible you could just not be seeing it. But I’m interested to see what our health expert has to say -- she can probably speak to this better than I can and she’ll let you know if you should go to your healthcare provider.