I want to use XPath expressions in Selenium code, but I don't know how to get it from a site's HTML code.
I'm using the Google Chrome web browser.
The easiest way is to inspect the element you want to get the XPath expression for. Then click on the highlighted code and Copy → Copy XPath which give you a full XPath patch you can copy to your code. For Firefox, you should install Firebug or another extension like this.
As neliCZka suggests, you could also try to build a relative path if its possible by trying to find the proper XPath in the search bar in Chrome after inspecting the element.