I have actually pretty simple question. Having new sitemap every week and get it indexed immediately. How can I guarantee that the new fresh content will be indexed immediately by Google without building new sitemap. I suspect there are should be kind of trigger to index just one particular URL that will be working with huge sitemap created early.
There is no way to guarantee it. Google will crawl your site whenever it feels like it. Typically sites that are updated often and have a lot of quality links are crawled more frequently then those that do not.
In response to your comments: Your sitemap should reflect all of the content on your website that you wish to have crawled and indexed. So if you keep adding pages to your site then your sitemap should grow accordingly.
Like I said in my answer above, Google isn't going to find your changes or update its index quickly for a site like yours. You need to be patient and continue to add content and seek links to your site. Google will eventually find your changes and update its index accordingly. If and when this happens is beyond your direct control.
The sitemap isn't all that useful unless you have over 1000 pages or so, so large that Googlebot isn't likely to crawl it all at once. As long as you have clear navigation and URL structure, Googlebot will find your content, new and updated, when it crawls your site. If your site appears more active, it will get crawled more frequently than a less active one. In other words, don't worry so much about the sitemap unless you have a highly complex or large site where Googlebot will need help in finding everything.
You can use pinging to alert search bots of new/updated content. Many CMS scripts have this built into them. You only need to ping once since additional pings are ignored.
It's also effective for indexing purposes to use Twitter, Facebook and other social media to highlight new content. If you can get other people repeating your new content links, so much the better.
Lastly, getting strong, long term, links to your content is essential to having your site rank well and for it to be crawled more often.