I'll try to help.
How do I get an avatar? There's one I'd like to use and I know where it is, but how do I make that image an avatar?
What you need to do is cut out the part of the image that you want (or use the whole image if you'd like) and resize it. You can do this using programs like Photoshop. You can do it with MS Paint but it'll turn out choppy and..yeah. When resizing make sure it is either below or at the size limit and the right file format.
To find out what is accepted, go into My Controls and select Edit Avatar Settings. Then you must upload the image to a file hosting site. Photobucket is a nice place, and easy to use. Once you have the URL (the link address) go into My Controls and then Edit Avatar Settings and post the link in "Enter a URL to an online avatar image". Then scroll down and click Update Avatar. If done right, you should have the image as an avatar now.
How does fanfiction.net work? I'm planing on writing a fanfiction there instead of here for reasons I'll explain later and I'd like to know how that place works before I join.
Oh dear, how to explain this... Well you already know the site is basically a large archive of fanfics right? Well with FF.Net, obviously to upload any stories you must first register, in which an e-mail address is obviously required. The e-mail address is needed so you can be notified of certain things, which you can modify in your profile. Anyway, in order to post up a story, you must first put the file in Documents. This is a part of the site where you can store a number of text files for a certain amount of time.
Once you have uploaded the file here, then you go into Stories and select New Story, and then click Guidelines. Make sure you read them so you know what is and what is not allowed on the site. You do not need to read this everytime after this though. After that, click "Yes, I have read and agreed to the content guidelines." Near the top, try to find "New Story" and it'll take you to a page where you must select the category.
After selecting the categories, it'll take you to the page where you put in the name of the story, the description, the rating, the categories, the language, and then there should be an area where you can put up the documents you uploaded previously. Select the one you want, and then click Submit Story.
Is that all you needed to know about FF.Net?
