Marrying a Japanese

What documents we need (as a foreigner) to get married in japan?

My partner is a Japanese, does it mean I would lose my nationality if I marry him or I can keep mine and he keeps his?

You would need to check with the city hall at which you plan to report your marriage in. Likely you'd need to get a certificate of your eligibility to marry, through your embassy in Japan.

When you get married, no, neither of you will lose/change your nationality. You keep yours, he keeps his. You will be eligible to apply for a Spouse of Japanese National visa so that you can stay with your spouse in Japan. Still you keep your nationality.