Hello shailendra, Google Webmaster tools does not create sitemaps. Try to use any external sitemaps generator.
php, perl, python scripts all third party. not google http://code.google.com/p/sitemap-generators/wiki/SitemapGenerators i prefer XML-Sitemaps Generator in Online Generators/Services list, but its limited to 500 urls
you can't create sitemaps within google webmaster tools. You can only use it to submit it. If you use wordpress, then there are plugins to create the sitemap. Search! If you use blogger, just use your feed url and submit it.
try with this one if you're familiar with php www. phpclasses. org/browse/file/21551.html (there's some minor error in code, output headers, but it can be easy fixed...)
I suggest using xml-sitemaps as well. there have great free sitemap generator that works for < than 500 pages. if you have more than 500 pages you can buy their standalone full version. also have you tryed gsitecrawler? Are you creating a sitemap for an ecommerce site or cms such as wordpres/joomla ? let me know i can suggest a couple good paid and free sitemap generators.
You can check this XML sitemaps tutorial overview on how to create, upload and submit sitemaps as well as pinging search engines when sitemap changes.