I am used to insert dummy data in test classes,but here I dont know what to do,for which object do I need to insert dummy records. Please Help

@RestResource(urlMapping='/v1/sample/')
global with sharing class sample {
    @HttpPost
    global static void sample(){
        RestRequest req  = RestContext.request;
        String docUpload=req.requestBody.toString();
        system.debug('*****docUpload*****' +docUpload); 
        String username;
        String password; 
        try{
            Id d;
            system.debug( JSON.deserializeUntyped(docUpload)); 
            requestWrapper jsonParsed =(requestWrapper) JSON.deserialize(docUpload,requestWrapper.class);
            username = jsonParsed.username;
            password = jsonParsed.password;          
            if(String.isNotBlank(username))
            {
                String username1 = 'thilak';
				String password1 = 'thilak';            
                username1 = username;
                password1 = password;
                RestContext.response.responseBody =Blob.valueOf('test');
                RestContext.response.statusCode = 200;
            }
            else
            { 
                RestContext.response.statusCode = 201;
                RestContext.response.responseBody = Blob.valueOf(JSON.serialize('bitmap not coming properly'));
            }
        } 
        catch(Exception e) {
            RestContext.response.statusCode = 400;
            RestContext.response.responseBody = Blob.valueOf(JSON.serialize('Something went wrong'));
        }
    }
    Public class requestWrapper{
        Public String username;
        Public String password; 
    }
}