A lot of new faces have been showing up, and some have never used a forum before. So, I decided it might be a good idea to make a guide on how to create/implement an avatar and/or signature.
Signatures
Often called "siggy" for short. It can be just text, or an image, or a combination of text and images. People often "make siggies", but what they're really making are banners. A banner is a (usually) small image set within a rectangle that is wider than it is tall.
To create your Signature:
Click your name in the upper right corner of the forum.

Signature is the first option, so simply enter what you want in the box.

HTML is a code system that uses tags <> to hold code. The default code in this forum is BBC, so HTML code is visible unless you enter it by clicking on the HTML button on the blue bar above the text entry box when you're posting.

Most HTML tags MUST be closed in order for them to work. </>
Common HTML tags:
<b> This bolds text </b> (You need to end this.)
<br> Inserts a line break (like pressing "Enter" on a keyboard)
<p> This starts a new paragraph This is very useful for splitting up your text
<i> This makes text in italics </i> (You need to end this.)
<u> This underlines text </u> (You need to end this.)
<font size="1">type your text here</font>
- Size 1 text
- Size 2 text
- Size 3 text
- Size 4 text
- Size 5 text
- Size 6 text
- Size 7 text
<font color="red"> text here </font> would appear as text here.
<img src="IMAGE URL"></img> THE QUOTES ARE VERY IMPORTANT!
You can also add a border around the image by adding <img src="IMAGE URL" border="2">
Border="0" no border
Border="1" thin border
Border="2" thicker border
<a href="PAGE URL"></a>
You can create a linked image by inserting the code for an image <a href="PAGE URL">here</a>; doing so will make it so that the image can be clicked on to take you to the page you're linking with.
<div align="center"></div>
<div align="left"></div>
<div align="right"></div>
<div align="justified"></div> (This one is only useful if you're working exclusively with text)
Since BBC code is what translates in posts, I can't display the codes here. However, click here for Wikipedia's list of BBC codes. ;P
Wikipedia doesn't have everything, so here is another source that has some BBC codes Wikipedia left out. ^^
After you've created your signature, click "Save Changes".
Avatars
This forum pulls an image from your computer to use as your avatar. If you wish to pull an image from a website, you must first save it to your computer.
From the same area that you set your signature, you can set your avatar.

Click "Browse" and find the image you desire, then click "Update". Sometimes it takes a while for your new avatar to show up. It says it can take up to 10 minutes, but I've had it take longer. The thing is, while you see the previous image, OTHER people see the new one. -shrug-
I hope this helps! If anyone has anything to add, please feel free to let me know, and I will update as needed!
Update(3/17): I forgot to mention that since characters are limited in signatures, it's advisable to use BBC instead of HTML because the coding for BBC is shorter.
Update(3/18): I realized I forgot to explain how to do links, so I added that. :)
Update(3/31): I've had some people asking me how to make a siggy, so (sorry if it's big):

Phoneuma:Hyperlinks to Trickster Forum Posts
Due to the limitations on sig length, every character counts. To add a hyperlink, you would normally have something like:
<a href=http://forum.ntreev.net/trickster/forums/threads/14676.aspx>
<img=http://www.imageshack.us/mypics/apicture09x.png></a>
Deleting the http:// from each of those links forces the forum to append those URLs to the current thread's URL, which leads to "Page Not Found" errors. That example would also take up a sizable chunk of your permissible siggy size. However, since you're hyperlinking to a post in the same forum, you can take advantage of the appendix function.
If we tried to use our current hyperlink sans http:// while on post 17834, the forum would reference the following URL:
http://forum.ntreev.net/trickster/forums/threads/17834.aspx#forum.ntreev.net/trickster/forums/threads/14676.aspx
This is definitely not what you'd want. You could try deleting all but the post's number, but that would yield the following:
http://forum.ntreev.net/trickster/forums/threads/17834.aspx#14676.aspx
Knowing anything after <a href= without an HTTP:// preceding it will be appended to the current post's URL, all you need to worry about is getting your hyperlink to shift the focus from the current URL to the parent URL. What is the parent URL? Let's take the above example and break down its structure.
HTTP:// Hyperlink call
forum.ntreev.net Domain name
trickster Great-grandparent URL (in the case of this example)
forums Grandparent URL
threads Parent URL
14676.aspx Child URL
In this case, we want to reference the parent URL, which is threads (we can leave off the slashes). To do this, you need to know a little coding trick used in DOS. In referencing paths, periods have a special function. A single period followed by a slash will reference the parent, while two periods will reference the grandparent. If you needed to, you could combine them. For example, ./../ would reference the great-grandparent. Or ../../ would reference the Great-great-Grandparent (the Domain in this case). Now you can cut out all that extra URL from your hyperlink!
<a href=./14676.aspx><img=http://www.imageshack.us/mypics/apicture09x.png></a>
Voila!