You need at least a BA (Bachelor's) in any subject to quality for a normal working visa. If you have a BA, you would know it, so I'm positive you don't. Once you have a BA, you need to find a company that will hire you and sponsor your visa.
There are many kinds of other visas, but they all require many various things.
http://www.mofa.go.jp/j_info/visit/visa/long/index.html
Here's a detailed list of what each visa requires:
http://www.juridique.jp/immigration.html
If you have a normal university degree (and papers to prove it) and speak good Japanese, you might quality for the "International Services" visa.