To add field navigate to (as an administrator):
Settings -> Customizations -> Customize the System
Expand Entities -> Expand Account -> Select fields -> New and assign a name and choose a type of Option Set
Add the values then Save and Close
Click back on Account on the left and then Publish from the top.
Multi-select lists aren't supported out of the box - you would need to do a little work with some JavaScript to make this happen. If you think you might only have 5 or 6 you could also create individual fields for each which could be displayed as a checkbox or Yes/No radio button type field.
What you are describing if fairly common - if you need to record multiple values and are just starting out I'd recommend creating multiple fields.