The Site Column helps you by providing a reusable column within your site and sub sites. Once you create a site column at our top level site , we can use that site column in the sub sites as well. Go to the  Site Settings   Next Under ” Galleries” , click  “Site columns”   On this page that is displayed, a list with all existing site columns is displayed. Click “Create” button  to create a new site column.  

• This is the Name and Type section, type the name which one you want in the Column name text box.
• Under The type of information in this column is, select the type that you want to appear in the column.
• Specify the site column  group, here you can select exiting group or can create new group.
• Depends on the type of column what you selected, more options may appear in the Additional Column Settings section. Select the additional settings that you want.
• To add the column to the default view, which people on your site automatically see when they first open a list or library, click Add to default view.
• Click OK

  If you Scrolle down can see created site column   Now create custom list( previous  post  how we created custom list in our sharepoint site), and then Go to the list Settings.   Click on Add  from existing site column   Select Group name  then you can find our site column(FirstName)  and add and Click ok.   We can see our site column added into our list