In the Customization menu, you can customize your avatar with different items that you can collect by progressing through certain leagues each season.

The customizations are, in order : Hairstyle, Chest, Girl 1, Girl 2, Backgrounds, Sex Poses, and Titles. After selecting one of these customization types, you can use the arrows to the left and right of the screen to cycle through different customizations.

Aside from unlocking customizations through seasonal league progression, some can also be purchased with Rubies, as seen below.  Rubies are earned from chests and other seasonal rewards.