Read the sizing guidelines and size charts at http://sizecharts.livejournal.com/ There are other charts at the Community FAQ if you don't like that one.

First, let me just say that I wish to god I had known about menstrual cups when I was 13 and had just gotten my first period. So kudos to you, starting out so young, I think you will save yourself so much grief in the long run. Well done!

Second, have you used tampons before? If not, and if this would be your first time using an insertable menstrual product, I've heard that the size small MeLuna cup is a great cup for very young teens to start out with (the regular small, not the "shorty" small - MeLuna has a zillion different sizing options).

I might suggest ordering two of the size small MeLunas - one in the "classic" firmness and the other in the "sport" firmness, which is a little firmer. Firmer cups are generally easier to get open inside you, which might be good for you, if you're just learning. But on the other hand, sometimes if you're really crampy, a slightly softer cup can feel more comfortable - so maybe starting out with both of those would give you the flexibility to accommodate yourself on different days. If you're able to purchase two, of course.

I would worry less about what your flow is, and more about getting the right fit for your anatomy. It would probably be better for you to get a smaller cup that's comfortable to insert, even if you have to empty it more frequently.

If you haven't done so already, I'd also encourage you to use either your index or middle finger to try and locate your cervix. How high/low it sits will probably eventually influence the types of cups you buy in the future (i.e., if your cervix sits low, you wouldn't want a super long cup, but if it sits pretty high, you might not want to trim off the stem too hastily). And whether or not your cervix points to a certain angle will slightly influence the way you insert your cups.

This may take some practice, so please be patient with yourself. I only just recently figured out that my cervix points slightly to the right, and I'm 28 years old! And like I said at the beginning, if you've never used an insertable menstrual product before, the MeLuna small is probably a pretty safe place to start.

I could easily go on about this, but this is already getting to be a long post - but if you have any questions about what I've written, I'll be keeping an eye on this thread so I can answer them. :)

I would be VERY careful about suggesting a short (low-cervix-friendly) cup to a young, childless, fit teenager. All those factors are more likely to mean the OP has a HIGH cervix and will have difficulty removing those Melunas, particularly if they are firmer and cause more suction. I was over twice her age when I tried cups initially--also Melunas, both Classic firmness--and I'll never get those 3 hours back spent trying to remove them.

I always recommend the small Si-Bell as a cup for first timers because the stem is long enough not to get lost inside a high cervix vag, but it's thin enough to be comfortable and can be easily cut to any length (or cut completely) for someone with a low cervix, and the cup body itself is short enough to fit a low cervix user comfortably. With the labia fold, it won't be any wider than an Ultra tampon, and despite the softness, its design allows it to pop open easily.

If you don't already, you might consider trying applicator-free tampons as a way of getting more familiar with yourself (since you're waiting to get a cup). I found the transition to cups really quick and easy, partly because cups are awesome and partly because I'd gotten that experience -- so while some things were different, overall, I knew what to expect.