Hypereddie

You must reach the Muffin rank (5 posts) to be able to edit your signature.
 
Once you reach that rank, go under your settings and click on the tab labeled "Signature"
 
 
 
 
Once there you can edit your signature to your liking.
 
Answer taken from: http://mlpforums.com/topic/67082-signatures/

Compiling all valid answers into one post.
 
 
 

Compiling all valid answers into one post.
 
 
 

One way to do it is by simply doing this:

Another way to is by using the editor. 
Click on this icon () in the editor and a popup menu will appear that looks like this:

 
In the popup menu, select spoiler from the dropdown menu. A text box will now appear below the dropdown menu, it should now look like this:

 
Enter the text you want to put in the spoiler in the text box and click ok and your done!
 
I'll leave this open for 24 hours so you can reply if your having any problem with putting text in a spoiler. After that the topic will be locked

This is correct.
If your thread is discussing season 4 and referencing Equestria Girls for predictions and why Sunset Shimmer might be in season 4 and future episodes, then it belongs in the Show Discussion section.
 
If your thread is discussing the Sunset Shimmer character in Equestria Girls overall and referencing season 4 to mention she might be in season 4, then it belongs in the Equestria Girls section.
 
The difference between the 2 being, your either focusing on the prediction and season 4 or your focusing on the character Sunset Shimmer overall.

Pretty much this. When you break one of the global rules, a global moderator or an administrator may issue you with a warning.  
Heres a quote from the FAQ: