Should I add a Site Map to my website? I was told that a site map can help me get better rankings in Google. Is it true?

Yes, it is always better to add sitemap into your website.Sitemap helps user for easy navigation and search engine to find your pages more in number.
There are two types of sitemap mainly used by webmaster : textual and xml both are useful.

If you website have 20-30 pages then I think sitemap is important for your website because sitemap help the users in finding your pages.But If your website have only 5-6 pages then sitemap is not really important.

Yes, most definitely, you should because an HTML sitemap:
- Ensures improved search and navigation by human visitors
- Improves user experience thereby reducing bounce rate
- Helps you distribute links to various pages of the website

In addition to adding an HTML sitemap to your website, you must also submit an XML sitemap to search engines to ensure better visibility and quick crawling.

S. Definitely you get better ranking by adding a sitemap. Add sitemap.html and also sitemap.xml.

yes , site map is important thing in website for user as well as for crawler.site map help user to find pages in your site.

You might want to check out this article describing all different sitemap file kinds:
http://www.microsystools.com/products/sitemap-generator/help/website-sitemap-kinds-comparison/

Site maps are a great tool for making search engines crawl through your website in a much more efficient manner, thus helping index all of your pages faster and getting you more traffic, FASTER! All major websites have them, and when a search engine crawler visits it and sees all the links within that website it gives the search engine the "green light" to start crawling them. Just like a geographical map, a site map helps the user find where they're going.

Always add a site map to you site.

Sure! You need 2 types of sitmap.
- A sitemap  with clear links for human visitors
- A XML sitemap for crawlers like Googlebot.

Site map is again of two types one is sitemap.xml and other is sitemap.html, sitemap.html is used for the user purpose and sitemap.xml is used for search engine purpose, so it doesn't help you in Google ranking but telling that you do have these many pages in your website.

You can add html sitemap on your site

Yes, it is always better to add sitemap into your website.Sitemap helps user for easy navigation and search engine to find your pages more in number.