How can a new Muslim declare his conversion to Islam and change his name on official documents?

Congratulations on your reversion to Islam. To declare your Islam, go to the mosque and inform the Imam so that it is known you have become a Muslim. As for changing your name on official documents, it is not obligatory unless the name is forbidden (such as names that indicate worship of other than Allah). However, it is preferable to improve it to an Islamic name. You can seek assistance from local Muslims to learn about the official procedures. Postponing this procedure is permissible if you fear harm or if there are difficulties.