The layouts folder can be found at the following location
%ProgramFiles%\Common Files\Microsoft Shared\web server extensions\15\TEMPLATE\LAYOUTS
The COREV15.CSS file can be found at
%ProgramFiles%\Common Files\Microsoft Shared\web server extensions\15\TEMPLATE\LAYOUTS\1033\STYLES
Were 1033 is the installed language folder.
You can register your CSS for example below the default corev15.css like this
<SharePoint:CssRegistration Name="Themable/corev15.css" runat="server" />
<SharePoint:CssRegistration Name="/_layouts/15/styles/<youre css file here>" runat="server" />
There is one more property of a SharePoint site called AlternateCssUrl. You would use this property if you don’t want to create a MasterPage or a Theme, but just apply a skin on the SharePoint default look – by specifying a value for this property, you make SharePoint include your style sheet in its pages.