@camcraw As others noted I'd also recommend looking at NISA to save the tax (or iDeCo, if you are okay to only receive the money after retirement) and invest through those into some low-cost, diversified option, like ETFs.
Overall there are only a few certainties in investing, and one of them is that diversification brings down risk. To give you an example: you like cars, so you invest in Volkswagen, but then they get into a scandal about their emissions and have to recall bunch of cars and pay fines making their stock value drop. Instead if you'd also buy stocks in all major automakers (e.g. Ford, Toyota, GM, Honda, etc.) then regardless of how the individual companies do, as long as people keep buying cars the overall investment will do well. Of course manually buying many stocks get complicated and expensive (transaction fees add up) and that's where ETFs (Exchange-traded funds) come in: companies like Vanguard buy a group of stocks and offer pieces of this bundle for sale, which then get traded similar to how normal stocks are traded. For example Vanguard's Total World Stock ETF (VT) includes stocks from 95,000+ companies worldwide weighted by their size (so e.g. 3% of it is Apple, but it also has a tiny portion of JP Post Bank stock). These ETFs charge a yearly fee (while holding individual stocks usually don't cost anything), but with Vanguard ETFs it's usually pretty low, e.g. the VT's fee is 0.07%/year.
You should also note that dividends from US-based stocks and ETFs get taxed in the US at 10%, which is not a problem if you need to pay taxes on them in Japan (as you can deduct the tax paid in the US from your Japanese taxes), but if you are using a tax-free account (NISA, iDeCo), then a non-US ETF would save that tax.
As someone starting out in investing I think the most important part is to take it slow and get yourself used to seeing the ups and downs of your portfolio. Markets move a lot in the short term, e.g. one day your investment might be up 5%, then next week it's down 3%. It's much easier to handle the down if the -3% only means you are down 300 yen as opposed to 30,000 yen. If you are investing regularly (let's say monthly) then it will help you see the market being down as things are on sale too.
Also note the foreign exchange risk. E.g. if you convert 13,312 yen to $100 now, buy 1 stock for $100. In one year the stock goes up to $110, but meanwhile the yen went back to 110 yen/$, so now if you sell and convert back you get only 12,100 yen. But if the yen goes to 150, then you get 16,500 yen. Investing in domestic stocks removes this risk, but Japanese stocks have historically offered much lower returns than foreign ones.
As a final note: this year has been turbulent, and no one knows what the next years will bring. We might be heading for a recession, Word War 3 or a quick economic recovery and continued growth. In these times it is especially important to decide and stick to your goal: if you need this money in a year, then don't invest it into risky things like stocks. If you are fine not touching this money for 5-10 years, then invest it, but keep this time frame in mind when the market goes up and down. Historically stocks had a positive return in the long term, but there have been multiple periods when in the short term they lost parts of their value.
