See if it has contact details and try to return it to the person. They will likely tell you to keep it. If you do due diligence to take the moral high ground, in my life experience it will really help define your character for years to come.
Finding something like this seems like a treasure at first. At your young age Rico, you likely do not see the ethics very clearly, at least when I was your age, I certainly did not have a very good moral compass in an area like this.
Over time, if you just keep it, you will experience losses in life where you will remember what actions you took and likely judge yourself accordingly. This is one of those rare circumstances that is likely to stick in your memory for far longer than the noise of daily life in strange ways. In the decades to come, this perspective will take on more meaning. Don't be too disappointed either way. This advice may be of greater value in the end. Trust me, I've gone both ways on the issue and experienced the loss. Doing the right thing for a stranger is ultimately a gift to yourself.