How can I create custom object using the Metadata API from Apex?

Go through http://salesforce.stackexchange.com/questions/20763/creating-a-custom-object-using-rest-api

Hi Piyush,

Try this:

HTTP h = new HTTP();
HTTPRequest req = new HTTPRequest();
req.setMethod('POST');
req.setHeader('Content-Type', 'text/xml');
req.setHeader('SOAPAction', 'create');

String b = '<?xml version="1.0" encoding="UTF-8"?>';
b += '<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">';
b += '<soapenv:Header>';
b += '<ns1:SessionHeader soapenv:mustUnderstand="0" xmlns:ns1="http://soap.sforce.com/2006/04/metadata">';
b += '<ns1:sessionId>' + UserInfo.getSessionId() + '</ns1:sessionId>';
b += '</ns1:SessionHeader>';
b += '</soapenv:Header>';
b += '<soapenv:Body>';
b += '<create xmlns="http://soap.sforce.com/2006/04/metadata">';
b += '<metadata xsi:type="ns2:CustomObject" xmlns:ns2="http://soap.sforce.com/2006/04/metadata">';
b += '<fullName>sample__c</fullName>';
b += '<deploymentStatus>Deployed</deploymentStatus>';
b += '<description>created by the Metadata API</description>';
b += '<enableActivities>true</enableActivities>';
b += '<label>sample Object</label>';
b += '<nameField>';
b += '<displayFormat>AN-{0000}</displayFormat>';
b += '<label>sample__c Name</label>';
b += '<type>AutoNumber</type>';
b += '</nameField>';
b += '<pluralLabel>sample Objects</pluralLabel>';
b += '<sharingModel>ReadWrite</sharingModel>';
b += '</metadata>';
b += '</create>';
b += '</soapenv:Body>';
b += '</soapenv:Envelope>';

req.setBody(b);
req.setCompressed(false);
req.setEndpoint('https://na12-api.salesforce.com/services/Soap/m/25.0');
HTTPResponse resp = h.send(req);
System.debug(resp.getBody());