You should first get Affidavit of Single Status, then the Affidavit of Single Status and divorce certificate should be certified/authenticated together, as for the detailed procedures, please refer to http://houston.china-consulate.org/eng/vp/fhjh/

Then after arriving in Changsha, please make registration with your Chinese girlfriend, then please find a job becaue marriage with Chinese citizen doesn't mean that you are permitted to stay in China for a long time, you can apply for multiple-entry L visa for family visit, but the each stay is less than 180 days, the best way is to find a job, then apply for work visa, then get a residence permit. If your marriage lasts 5 years and annual stay in China is not less than 9 months, you can apply for Permanent Residence Permit (China Green Card)