Create sitemap. FTP upload it. Submit it to Google Webmaster tools. For more information you really need to give more information (like website platform etc.)
If the website is run on CMS platform then you have plugins to add xml/html sitemaps, whereas if you have static or dynamic websites then there are two ways to add xml sitemap, first, through hands-on coding and second, through xml sitemaps generators. I recommend xml-sitemap website to create sitemaps. They have free version to create sitemap for 500 pages website, just enter your url and voila! you have a sitemap.xml file, which you need to put in the root directory of your site i.e. using FTP.