Click on your user name on the right side of your screen and click the "my store" link.
Click on the tab marked "products", find the product you'd like to add an option to, and then click "edit."
Now look for "options" on the left side, click, and then select "+ New Option."
Here, you can put in all of the details of your option. To add more than one option, just click the bottom right button marked "new option."
Here is an example of a filled-out form:
And here is what buyers will see on my page:
The base product must be the lowest price, and then in the option settings you will add how much more it will be than the base price. If all the options are the same price as the base product, you can leave 'Additional Cost' blank.
If the 'This option is required' box is checked, then the user must select one of the items in the dropdown before adding the product to their cart.
Setting A Default Option
You can set one of the options as the default by checking the "Select this choice by default" box. Unless the buyer changes it, this will add that option to the cart when the buyer clicks on add to cart.
You do not need to fill out the 'Quantity' for the options, but if you do you will need to make sure that the quantity set on the main product info page is enough to cover them all. If you have 3 x option A, and 3 x option B, then the product quantity will be 6. If you have a product quantity of 3, then only 3 of either option can be sold before the product goes out of stock.
You can have more than one dropdown if you have more than one variant that the buyer can choose. e.g.