Your user page is probably still empty, in this case the editor is automatically opened. If not, click the 'edit' button at the top of the page, third from left.
Now you are editing your own user page! A few advices before you start:
For those who have never made a User Page before on another wiki, it is advised to start with an existing template or user page. Either find a user page which you like the look of and copy-paste the code over to yours, editing it in the right parts to make it relevant to you. Alternatively you can use this exemplar User Page template, instructions on how to customize it for yourself are listed on the edit screen.
If you have made a User Page before on another wiki (such as GWWiki or GuildWikia) then you can copy-paste the same template here. Be careful to change internal links to external ones. You will need to re-upload any pictures you want to include on the page.
Once you become more fluent in how to edit these types of pages you can create your own designs.
Wikicode is a programming language designed for wikis.
Things that work
Things that don't work
You may find that some things don't work in wikicode. For example: