if you want your own hotspot I recommend setting that up before you go. I don't know if there are many options if you wait until you arrive. Here's a post about my most recent experience renting through Global Advanced Communications:
Data and phones | Japan Forum
7-11 and some other convenience stores offer free wifi. Starbucks as well, though it might need a little setting up. I'm not sure if McDonalds does but I would think so.
One time I flew in, there was a free wifi hotspot service for tourists that required you to download an app. It was clunky and I never got it working properly, despite having an attendant help me set it up. I would connect but I couldn't actually use the data. Fortunately I didn't really need it. This article covers that and some other options:
Internet Access in Japan
All that being said... you're a teenager going on vacation to Japan. Get your nose out of your phone and pay attention to your surroundings. See it not only as a vacation from your home country, but take a little break from technology as well. Use it to keep in touch with people but don't waste too much time playing games on it, or you might miss out on the experience.