Clicking on the my preferences link in the upper right while logged in then click on the Skin button to change your skin. You can also preview the skin by clicking the (preview) links next to each skin.
You can make changes to the current skin's stylesheet file (CSS) by creating a subpage of your userpage, "User:Yourname/monobook.css" for example. This requires your site admin to have enabled this feature -- if it is, you will see advice text at the top of your custom CSS page about clearing your browser's cache.
- Manual:Skins - in the technical manual (for server administrators)