For a student visa you need to have a firm plan to come here to study, at a school that is recognized by the government as a legitimate school (in other words, you can't come here to study anime in your friend's apartment). The visa is the government's endorsement of you and your school's plan to provide you with some education.
It is more or less the same for a cultural activities visa (and a work visa, for that matter). You must have a plan that includes sponsorship by a genuine provide or cultural activities. Once you have the plan, the government then either endorses it and grants you permission (the visa) or not.
For work, you have to have a job lined up and the specifics of the job have to be clarified by a contract between you and your future employer. Once you have this, you apply for the visa.
You cannot apply for a visa with a vague plan of living in Japan and then figure out the job, school, or activity when you get here. Even if you have a detailed plan, you cannot apply for the visa if that plan isn't somehow validated by the institution in Japan where you will study, train, work, etc...
It is a common misconception, and a common question: I want to live in Japan, which visa do I apply for? As above, Japan only allows residence here as a condition of employment or school (or if you are a dependent here). So you need to sort out your activity and the sponsor first.