apply for work visa

You should first prepare the relevant documents including your passport and its photocopy, the application form, a passport-style photo, and the alien work permit from your employer.

i already have relevent document. my employer email me a contract paper for job. do i need other document for work visa？

Your employer needs to submit a petition to the local labor bureau and wait for their response first (providing that they are qualified to hire foreigners in the first place). You will not be able to start the application process until the approval from the labor bureau (and sometimes the foreign affairs office) is granted and a letter of invitation (or similar official document) is issued.