You could use product attribute for product variants such as Color, Size etc. In Catalog -> Attribute, you could add a product attribute to be shared by all the products. In "Add a New Attribute" form, fill all the field and click "Add" button. Here are some explanations to the fields.

• Attribute Name: the name that you assign to the attribute. E.g. Color, Size
• Required: if checked customer has to choose a value for this attribute before he could add the product to the shopping cart.
• Selection Type: only "Single Selection List" is available now. More types such as multi-select will be added in the future.
• Display Control: how different choices of the attribute are shown.
• Description: a brief description of the attribute

To associate attribute to the product, use Add Product or Edit Product. In Attributes tab you could associate attribute to the product. In Attribute column pick up a store wide attribute that you wish to assign to the product. Fill out other columns and click "Update" button.

Refer to Associate Attribute to Product on how to open product Attributes tab. In Value column enter a value of the attribute such as XXL if attribute is Size. To add more values, fill out more rows with different values.