Access a Records Management site just as you would any other Alfresco site, from the My Sites dashlet or the Sites menu.
Instead of storing your files in a document library as you do in a "regular" Alfresco site, in a Records Management site you file your records in the File Plan. And you can't edit the content of records; once they're in the File Plan record content is considered to be final.
You file records by adding them from your computer to the File Plan, or if you're in another Alfresco site you can declare a file as a record and it will be added to the Records Management site. Once files are added to the File Plan you can edit their metadata (properties) but you can't edit their actual content.
If you're new to Alfresco then take a look at Using Alfresco to help you get started.
For more details on setting up and administering Records Management see Administering Records Management.