The following article illustrates how to add a column to a Library
How to Add a Column to a Library
1. Select the Library that you want to add a Column to.
2. Click the Library tab to change the ribbon to view all settings for your Library.
3. From within the ribbon, under the Manage View section, click the Create Column button.
4. Type the Column Name within the Name and Type field. Under The type of information in this column is, select the information type that is going to be described in the column you are creating.
5. In the Additional Column Settings section, type a description in the Description box (this is optional). There is additional setting such as "require that this column contains information" and Maximum number if characters". Select the options that best fit your needs and click OK to add your new library column.