2.1.DS-2019 is proof of J1 status, most institutions and some companies may issue a DS-2019. Your sponsor (institution or company) will send you this form, you will need this form to apply for a J1 visa.
2. Only the institution or company you will work for can issue a DS-2019 to you, you have to report to them immediately after your arrival. You can't choose who will issue that form to you.
3. You don't need a waiver if you want to leave US after J1. But if you want to come in future, you have to stay in your homecountry (China if you are currently hold Chinese passport) for two years before you will be granted a US visa other than B2.