Hi,

Here im connected to hoovers by passing account name as input and got the duns and other fields as per my requirements. See the below code

global with sharing class HttpCallout_CLS{

String Accid = apexPages.currentPage().getParameters().get('id');

Public void sendRequest(){
    String env;
    integer j,k;
    HttpRequest req = new HttpRequest();
    req.setMethod('GET');
    req.setHeader('content-type','text/xml');

    req.setEndpoint('http://hapi.hoovers.com/HooversAPI-32');
    String API_KEY = 'XXXX'; // Use Api Key
    Blob headerValue = Blob.valueOf(API_KEY);
    String authorizationHeader = 'BASIC ' + EncodingUtil.base64Encode(headerValue);
    req.setHeader('Authorization', authorizationHeader);
    String CName = apexPages.currentPage().getParameters().get('Name');

              // **** removing special characters from a account name*****
    
    List<String> spl= New List<String>{'@','#','$','%','^','&','*','(',')','!','.','"','<','>','?'};
    List<String> li = New List<String>();
    String CompName='';
    for(j=0;j<CName.length();j++){
      li.add(CName.substring(j,j+1));
     }
    for(j=0;j<CName.length();j++){
        for(k=0;k<spl.size();k++){
            if(li[j]==spl[k]){
                 li[j]=' ';
                 break;           
            }
        }
    }  
   for(Integer l=0;l<li.size();l++){
       CompName = CompName + li[l];
    }

//constucting request body

 env = '<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:sch="http://applications.dnb.com/webservice/schema/">'+
            '<soapenv:Header>'+
              '<sch:API-KEY>xxxx</sch:API-KEY>'+
            '</soapenv:Header>'+
            '<soapenv:Body>'+
              '<sch:FindCompanyByKeywordRequest>'+
              '<sch:maxRecords>100</sch:maxRecords>'+
              '<sch:sortDirection>Ascending</sch:sortDirection>'+
              '<sch:keyword>'+CompName+'</sch:keyword>'+  // search keyword
              '<sch:searchBy>companyName</sch:searchBy>'+
              '<sch:returnSearchNavigation>true</sch:returnSearchNavigation>'+
              '<sch:orderBy>CompanyName</sch:orderBy>'+
              '<sch:searchNavigation>'+
                '<sch:employeeSearchNavigation>'+
                '</sch:employeeSearchNavigation>'+
                '<sch:ownershipTypeSearchNavigation>'+
                '</sch:ownershipTypeSearchNavigation>'+
                '<sch:locationTypeSearchNavigation>'+
                '</sch:locationTypeSearchNavigation>'+
                  '<sch:salesSearchNavigation>'+
                  '</sch:salesSearchNavigation>'+
                  '<sch:industrySearchNavigation>'+
                 '</sch:industrySearchNavigation>'+
                '</sch:searchNavigation>'+
               '</sch:FindCompanyByKeywordRequest>'+
              '</soapenv:Body>'+
             '</soapenv:Envelope>';      

     req.setBody(env);  // sending request to the API
    System.debug(req.getBody());     
   Http http = new Http();

   HTTPResponse res = http.send(req);  // getting response from the API

 System.debug(res);

Use parsing classes/techniques (XMLNode...) to parse the response

Hope this will helps you.