Action Details
Requirements and how-to articles for Steam's action:

Requirements
You need this to get it done:

Email Address
Enter the email address you wish to be associated with your Steam account. This is where your confirmation email will be sent during the account creation process, and is also used to confirm purchases and help you manage access to your Steam account.
Account Name
A unique username for your account (this is not publicly displayed) (maximum of 64 characters.)
Profile Name
Your publicly displayed username. This does not have to be unique. By default this is the first part of the email address you used to sign up. (e.g. an email address of "johnsmith123@emailprovider.com" would result in a default profile name of "johnsmith123") (must be between 2 and 32 characters in length, can include spaces and special characters.)
Avatar
An image to represent you/your profile on Steam.
Maximum file size: 1024KB
Source file can be any size as long as the file size is under 1024KB. Will be automatically resized to 184 x 184, 64 x 64 and 32 x 32 pixels.
Profile Summary
A description of yourself/summary of your profile (maximum of 4000 characters.) Markup tags can be used for formatting. Can use Steam emoticons.
Custom URL
Having a custom URL makes it easier to share your profile with others. Your full profile URL will be set out as follows: http://steamcommunity.com/id/[custom URL]
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

0
Steam Support - How do I create a Steam account?
Guide to creating an account via either the Steam client or the website.
Official
0
Steam Support - Steam Community Avatars
Information about uploading an avatar.
Official
Link missing here or found errors? Improve How-Tos