MediaWiki's interface, or skin, or even more simply, how it looks, can make or break how easy MediaWiki is to use.
MediaWiki comes with several skins by default, and below is a brief rundown of how useful they are to a beginning user.
Chick is basically a really lightweight, stripped down version of the Monobook skin with no sidebar. It has a lot of screen area for information, but otherwise is not very beginner friendly due to lack of features.
Cologne Blue has a small sidebar, a big blue header bar at the top, and a lot of content space, but that's about it for advantages for beginners.
Modern is a somewhat more useful skin that features small header bars, a non screen hoggy sidebar, and most menus are slightly boring but easily accessible.
Kinda dull and boring in terms of scheme (it's more about function than aesthetics), but it's a good choice for beginners. A few Gadgets have been made with this skin in mind as well that give it more user functions as well.
MonoBook used to be Wikipedia's default skin until vector came out. Very dull and businesslike in appearance, but it's very similar to Modern in being very functionally friendly with bars and tabs easily accessible, with a small but neat sidebar and fairly nice amount of content room.
Decent choice for a beginner, but there are better options. (Including vector, the smoothed and updated Monobook-like skin.)
Not a good choice for beginners to MediaWiki at all.
Nostalgia is basically a skin that emulates Wikipedia's appearance from 2001, and it has no sidebar, but has a lot of content area, with most of the bars and tabs and search boxes at the top and bottom of the skin.
Not a good beginner choice.
Simple is basically a stripped down version of MonoBook with more content area, but no header tabs and a very simple background scheme and sidebar.
Okay choice at best for a beginner.
Standard, also known as "Classic".
It's basically Nostalgia with a sidebar, and it's not bad, but there are better options for a beginner.
Vector is the current skin used by Wikipedia, and is probably the best choice for beginners due to it's smooth look (like Monobook without the dull bulletpoints on the sidebar, and such others.) and easy accessibility of tabs, the search box, and other menus.
About the only bad thing is that the content area is slightly smaller than most other skins, but not by a huge amount.
It also has a companion extension, also called Vector, that makes the sections of the sidebar collapsible.
Some people have either created skins based off of existing MediaWiki stock skins, or have created entirely new ones that may be a use of to MediaWiki beginners.
It features an extremely nice looking interface, good looking and easy to use bars and buttons, and the sidebar features expandable menus.
Not included with MediaWiki by default, but is a good choice for any MediaWiki beginner who doesn't like or want to use Vector, though tweaking the sidebar and the other features is different than with most skins.
Foreground is a custom skin that, for beginners, is really good if your main concern is content space, as it compresses the side bar and most other menu that take up space on other skins into a series of drop down menus, leaving the content space to span the whole page. It also has large text, so it's easy to read, not to mention looks really good on mobile devices.
Unless you prefer sidebars and don't like drop down menus, this is a fairly good choice for MediaWiki beginners.
Oasis is not available for public use, but it's the current skin used by Wikia, and for any MediaWiki beginner who wants to establish a wiki on Wikia, here's the basics.
It has no sidebar, instead featuring the sidebar menu across the top of the screen. The content window is rather small due to the larger recent changes window and ad bars on the right, top, and bottom of the skin (some of this is hidden or minimized if you are a member of Wikia instead of being an anonymous browser), and the skin is stretched for widescreen users and will look overly large and require using scroll bars to view everything on small monitors.
In short, it is not very beginner friendly. Also, this skin can only be added to a wiki if it was made using FANDOM powered by wikia.