If I submit sitemap.xml to the Google webmasters, will it help me for my ranking?
There are no evidences that a Google Sitemap will directly affect your ranking.
However, it can help Google to crawl and index your page deep and complete which increases the possibility of coming in SERP's.
You only really have to submit a new sitemap when you have made major changes to your website layout.
Yes rightly said. There is no such evidence that xml sitemap helps in ranking. There are mix opinions on this but yes like others xml sitemap is good way to inform Google about the website pages and can help in indexing.
I think submit sitemap.xml may not helps to ranking but it helps to index your website pages.
No when ever site map is changed or any change in website only then submit it again.