This guide will help you get started if you don't know much about coding.
A big mistake many newbies make is using the Visual Editor. If you want to code, do NOT click the 'Edit' button. Instead click the triangle next to the edit button, and click "Classic Editor". Then, switch to source mode using the tabs on the top. This is where you code. The Visual Editor does have a source mode option, but it will sometimes screw that up and display the code and not the effect of the coding. Save yourself the hair-wrenching frustration of coding properly by seeing your coding displayed as regular text and use the Classic Editor. A good thing to do is to set your default editor to source or classic editor in the Editing section of your preferences.
Close off Your Coding!Edit
When I was new to coding, I made this mistake. You must close off your coding. At the end of your sig, user page text, or whatever you are coding, add which one of these was at the front: div, span, or something else. If you don't close it off, your page will be a mess, and everyone typing their message or something will have it coded with your coding! So it is very important to close it off!
Here are links to pages on the wiki with basic coding, that is great for beginners.
Just to name a few! If you need a template, go to Free Designs, but be sure to credit the owner of the design!
You may not want to start on this until you are advanced in coding! But if you are looking for a challenge, here are some coding articles that may help!
Before You StartEdit
I personally think you should look at these articles before you start: Size
If you looking for examples, Free Designs is a good place to go!