「 I just can’t find a good example of what clothing I should wear when I go.」
If you're talking about personal clothing (i.e. not a school uniform or something to wear to an office job), you won't find any information on clothing you "should" wear, because honestly, there aren't any set rules for it.
If you're from the U.S., it's unlikely that your clothing is going to attract any attention, particularly negative attention, from teenagers in Japan. Especially since you describe yourself as "a modest person," I'm assuming your fashion sense isn't particularly outlandish by U.S. standards, and so it's unlikely to be seen as outlandish in Japan either. It sounds like your goal isn't necessarily to have Japanese people think of you as super-fashionable, but just not to make a negative impression, and so I think you'll be fine.
That said, here are some things you might want to keep in mind (though they may be things you already know and/or wouldn't be an issue because of how you normally dress anyway).
1. Japanese fashion tends to be, on average, a little dressier than U.S. fashion. That doesn't mean that Japanese teens all wear expensive clothing, but on average I'd say that their clothing appears more freshly washed/better taken care of than what I was used to in the U.S. Clothing that's damaged or dirtied (holes, tears, stains, faded fabric colors, severe wrinkles) generally isn't worn out in public.
2. Similarly, Japanese teens don't usually wear sportswear unless they're actively exercising or playing sports. So like, you probably won't see them wearing sports team jerseys, track pants, or clothing made of jersey-like material, as regular clothing when out and about. The same goes for sweatpants and, for the most part, sweatshirts - they're pretty much for exercising or relaxing at home.
3. For women, low-cut tops are rare.
4. Japanese fashion is very more accessory-focused, which also extends to wearing multiple layers, even when the weather isn't particularly cold. Having grown up in Los Angeles, if it's warm I usually wear only a single layer on top, but in Japan you'll see many people wearing over shirts, visible undershirts, cardigans, half-jackets, etc. Not wearing layers won't make you look slovenly or overdressed, though, so no one will react negatively to it.
In short, if you're an American teen who dresses modestly, I don't think you'll have any problems in Japan as long as your clothes are freshly washed and not worn out or severely wrinkled.
「Can anyone...give me any examples?」
Honestly, I'd recommend taking a look at Uniqlo's website.
https://www.uniqlo.com/jp/ja/While it isn't high fashion, Uniqlo is the most popular casual clothing chain in Japan. Wearing styles similar to what you see on their website won't have people in Japan asking you to coordinate their outfits for them, but it also won't have them laughing or glaring at you for dressing inappropriately. Basically, Uniqlo (or similar styles) is always a safe choice for casual social gatherings in Japan.
One last bit of advice: You said you'll be going to Japan for a year, and during that year, it's likely that you'll experience some changes to your figure/clothing size. This is pretty common for exchange students as they adjust to their new diet and lifestyle in Japan, and it isn't as simple as "everyone loses weight" or "everyone gains weight." Yes, portions are smaller in Japan...but there are also lots of all-you-can-eat deals, and almost every restaurant allows you to order extra-large portions too. Yes, there's a lot of rice and fish...but also an amazing number of awesome dessert cafes and sweets stalls. Yes, you'll probably walk more than you would in the U.S., but gyms are harder to find and usually don't offer as many programs/equipment. So don't stuff your suitcase with clothes that just barely fit or try to take every piece of clothing you'll need for the entire year, and expect to buy some things after you arrive and settle in to your new lifestyle.
