As the other comments have mentioned the hostnames in the config need configuring however alongside that there are quite a lot of things to consider for multi-site in Sitecore and the difficulty of getting it to work depends on if it was considered in the initial build or not.

I'll jot down some bits and pieces to consider, especially in the scenario where this is not a greenfield build.

Global Settings

Have an area of the tree for true "Global settings" across all sites in Sitecore and a separate one for each of the specific global settings, for example site logo etc.

Shared Components

Do you need global components and site specific components? If so split component folders to global and site specific too

Relative data sources

Check to see if any data sources need to be changed to be relative to the site you're in, useful when referencing site specific global settings.

SEO

For SEO you need to consider a separate robots.txt and sitemap.xml for each site

• There is a Sitemap XML module available on the Sitecore market place (https://marketplace.sitecore.net/Modules/Sitemap_XML.aspx), this will need to be assessed to ensure it works correctly in Sitecore 8

• There is a Robots.txt module available on the Sitecore market place (https://sitecoreclimber.wordpress.com/2014/07/27/sitecore-multisite-robots-txt/), this will need to be assessed to ensure it works correctly with Sitecore 8

Search

Ensure that Sitecore only searches over each individual site not all microsites - details on this are here (for SOLR)

• http://stackoverflow.com/a/33062404/861315

404 and 500 pages

Do these need to be different for each site? In which case this will need to be handled; pretty sure we used a module off the marketplace for this but I can't remember which; I'll update this when I track it down.

Media library

Organise the media library so there are different areas for each site

Security and workflow

Add security and workflow to lock down roles to access the correct areas of the site

Web forms for marketers

We didn't need to do this as the micro-sites didn't require forms however details are here:

https://doc.sitecore.net/web_forms_for_marketers/setting_up_web_forms/installing/multisite_implementation_with_web_forms

As the other comments have mentioned the hostnames in the config need configuring however alongside that there are quite a lot of things to consider for multi-site in Sitecore and the difficulty of getting it to work depends on if it was considered in the initial build or not.

I'll jot down some bits and pieces to consider, especially in the scenario where this is not a greenfield build.

Global Settings

Have an area of the tree for true "Global settings" across all sites in Sitecore and a separate one for each of the specific global settings, for example site logo etc.

Shared Components

Do you need global components and site specific components? If so split component folders to global and site specific too

Relative data sources

Check to see if any data sources need to be changed to be relative to the site you're in, useful when referencing site specific global settings.

SEO

For SEO you need to consider a separate robots.txt and sitemap.xml for each site

• There is a Sitemap XML module available on the Sitecore market place (https://marketplace.sitecore.net/Modules/Sitemap_XML.aspx), this will need to be assessed to ensure it works correctly in Sitecore 8

• There is a Robots.txt module available on the Sitecore market place (https://sitecoreclimber.wordpress.com/2014/07/27/sitecore-multisite-robots-txt/), this will need to be assessed to ensure it works correctly with Sitecore 8

Search

Ensure that Sitecore only searches over each individual site not all microsites - details on this are here (for SOLR)

• https://stackoverflow.com/a/33062404/861315

404 and 500 pages

Do these need to be different for each site? In which case this will need to be handled; pretty sure we used a module off the marketplace for this but I can't remember which; I'll update this when I track it down.

Media library

Organise the media library so there are different areas for each site

Security and workflow

Add security and workflow to lock down roles to access the correct areas of the site

Web forms for marketers

We didn't need to do this as the micro-sites didn't require forms however details are here:

https://doc.sitecore.net/web_forms_for_marketers/setting_up_web_forms/installing/multisite_implementation_with_web_forms