When you need to store custom data on contacts, opportunities, etc. you can create a custom attribute on the object type. First, click on the Gear button and then on Configure Attributes menu item.
Here you will see a list of all the attributes for this type. Click on the + button to create a new one.
Now give it a label and select the type. You can choose from a number of different types, including text, numbers, money, dates, etc.
If you choose text, you can also restrict what can be stored by selecting the Dropdown Menu option. Then you can enter each of the items you want. Just put each one on a separate line.
Once you've done this, click Create.
And here you can see the new custom attribute.