Hi all, I need your help. I've just created sitemap.xml using free online sitemap generator. Everyday I make new postings to my website. My question is: Will I have to my sitemap.xml regenerated and resubmit it to google webmaster tool?

Your info is highly appreciated.

You will need to regenerate sitemap from time to time, or use standalone php script (http://www.xml-sitemaps.com/standalone-google-sitemap-generator.html) and setup a scheduled task to automate that.
Google Webmaster tools will remember your sitemap and will re-download it every so often, You can go to Google Webmaster tools and resubmit if you like, but Google remember where it is, just update the sitemap.xml on your server.