Okay, so you've created a user, got logged in, and you're marveling at the wonderous complexity of the site and wondering how you can help. Well, maybe not, but I'm going to tell you anyway.
There are a lot of options in your little menu on the right. the one I'm going to help you with today is the Create Content link. When you click this link, you are presented with a list of content types similar to this:
Blog entry
Event
Group
Image
Page
Personal Page
Poll
Story
I'm going to try to explain the differences between some of these, since the content type you use to post something on the site affects how people will find it.
BLOG ENTRY
This is your personal blog. These are mostly of interest to your buddies, and they will gain more visibility as we get more people blogging on here. This is where shorter articles should probably go, too, unless they are meant to be of more permanent use, in which case you'd create a story, which we'll get to in a bit.
EVENT
If you create an event, it is added automatically to the calendar. Once you get in there, it's pretty self-explanatory. You pick a start date, end date, give it a title and a description, and submit it.
GROUP
This gets a bit more complicated, so I'd recommend familiarizing yourself with other content types first. When you're ready to create a group for whatever purpose (guild, fighting company, household, park, Amtgard Sluts Anonymous, etc.), it's very easy to create, but a little harder to understand what you've created. Don't worry, you won't break anything by creating one, but I'm going to put this all in a separate post at some point.
IMAGE
Every item you create is a "node." This applies to images as well. Basically, it's an upload form for a single image, and you can put it into a pre-existing gallery of your choosing. If you want a gallery created, please put a request in the request forum. I haven't yet figured out how to allow users to create their own galleries, so please be patient for now.
PAGE
I'd recommend avoiding this for the most part. This is for static pages you intend to have added to the site. This is NOT for articles, personal pages, or anything similar. It's more for menu pages or other perpetual content. If you want to create a page, go ahead and create it, and then ask a moderator to put it in a menu for you. Until it's in a menu, you'd have to link to it directly from another page.
PERSONAL PAGE
Know a little HTML? Even if you don't, would you like to create your own little AmtSpace page showing off who and what you are? This is where you do it. These show up on the User Personal Pages link off the Populace page. As soon as you create the page, it's life, and it can be found under your name in the list.
POLL
This is probably pretty self-explanatory. You can create a poll for anything. Really curious what brand of potato chips Amtgarders use? Here's a good, unscientific way to find out.
STORY
This is where most of your content should be created (saved the best for last here). If you want to write an article, background story, feast recipes, etc., this is where to do it. Stories appear under the User Stories link off of the Populace page for now. Just look for your name in the list. If it's really good, it may get promoted to the front page.
A note about attachments:
Attachments will be listed at the bottom of your content if you leave the List box checked next to the attachment name. If you uncheck this, the file will be invisible. The URL appears below the description after you attach a file, and you can use this as either an img: or a: tag reference if you know HTML. My advice is just to play around with it, have fun, and ask if you can't figure it out.
I hope this helps clear up some confusion you may have. Go post now, and ask if you have any questions or run into trouble.