Where to begin?
If you have motivation to contribute to Lexicanum, the big question is where and how. Lexicanum has grown quite big, and most important topics already have lots of good articles.
We would like to encourage our newcomers to start out slow. Look for a topic you are interested in, and read the articles that already exist. Click along. Sooner or later you will find a page you can add some information to. Off you go! Click "edit" and edit away! Save it! On to the next article.
By reading articles and source code (which is the text form of articles that appears in the edit window) and editing that source code you will learn pretty quickly how a Wikipedia article should look.
A little note: the source code of some articles looks pretty complicated at first. The reason for that is that it contains tables, and the method for writing tables is, well, advanced. Don't let that stop you! Just scroll down a little more in the edit area. After the table comes the actual article text, which is much easier to read and edit.
After that, create new content
On the other hand, everyone wants to contribute new content. One of the most common mistakes for beginners is starting after two minutes in the sandbox by creating five to ten new articles with only two or three words each in rapid succession. It is a big letdown to see these pages deleted in cold blood. However, that is what has to happen because we are trying to write an encyclopedia, something that can compare to the Britannica - well at least on Warhammer - one day.
So, Before you start on new articles: Please read our remarks on stub and tips for writing good articles. A new article should have some quality: a few lines are necessary and two or three are enough. One of the most important things is to always(!) write complete sentences. Ideally, the first sentence should contain the term that is explained. Articles like "Lexicanum: Its a online encyclopedia" don't help much, and copying your thesis is also not a good idea in most cases: Joe Average should be able to make sense of the articles. But don't be too shy; we don't expect your articles to be perfect.
That is why we recommend reading and editing first, so you get a feeling for our style before you start on new articles. Have a look on the lower half of the new pages page to get an impression of how new pages should look.
Now, if you want to start writing new articles: On the Portal:Lexicanum there are some topics for which articles are requested (remember: Really intend on writing a short article, not only three words!). Also keep in mind that a whole bunch of missing articles are on the Wanted Pages page.
Please experiment only in the Chaos wastes. Test masquerading as "normal" articles are deleted immediately. That means, you can't see if your test was successful, because an admin already deleted it. Furthermore, the admin had to spend some time and resources to delete that article, and because admins are friends, not food, you have to be nice to them by not causing them additional work.
Lists and placeholders
Creating lots of placeholders (articles that don't actually contain information) is another mistake common among newcomers. Also, often articles are created that contain only lots of links to sub-articles, which presumably is intended as some kind of subdivision - without some general information besides the links. However, content should, if possible, always be joined and not split over many small articles. So, if you have something to say on a topic, only create one article first. If it gets to big, you can always split it later.
... are one sentence mini articles with little to no content. These can be quite useful, but a little content is a must for any new article. On Lexicanum:Stub, there is more information on stubs, and also some examples of good, helpful stubs and bad, frowned upon stubs. For everyone unwilling to read that page now: Bad stubs are something like: "Valten - was assassinated" or "Kislev - a kislevite city in... Kislev". New articles of this caliber are quickly deleted, so keep in mind that new article should give people knowledge, not just drop some names.