Hi Pamela. Many things have changed, most for the better, I think. Start by clicking on the The Next Big Writer logo on the upper left of your screen. That will take you to your home page, which has lots of useful information as you begin to use the site regularly. On the upper right of the home page is an orange button labelled Conversion Assistance. That should answer many of your questions.
At the very top of the screen are icons for searching (the magnifying glass), reading content, reading group forums, and a menu button with your userid on it. That menu is important, as it gives you access to many of the features you would most want about the site. The Portfolio menu item is where you'll find your own work (books, poems, etc.). Click on the various items in the menu to see where they take you and explore a bit. It isn't too hard to get used to.
A couple of new concepts to be aware of inline reviews and groups. Most people love the inline reviews. You can select a piece of text in someone's story and drop a comment right there. No more constant scrolling up and down to copy/comment on specific items in someone else's story. Your inline reviews can be saved if you can't finish in one session, or submitted to make them visible to everyone else, including the author. Note that your feedback and the author's replies are now visible to everyone. Previously, the author's reply was private to just the reviewer.
Note that there are two kinds of reviews: regular and inline. You can leave either type of review for someone. Regular reviews are just like those on the old site. Inline reviews are as described above.
Groups are a new concept on the new site. As the name implies, they are for grouping related information together. For example, there is a Sci-fi group where you can publish and/or discuss science fiction work. There are also groups for fantasy, detective/crime, young adult, etc. You've already found your way to the Old Forums group, so you know how to navigate to groups. From a group's main page, you can get to the forum for that group, see content posted to that group, and see members of the group. Joining a group is easy. There's a button for it on each group's page. Most groups are wide open, and anyone can join by simply clicking on the button. Other groups are restricted, so I assume a moderator will have to approve your request to join. As far as I can tell, you can see the content of each group, including the forums, but you need to be a member to post. You can join up to 10 groups, which seems to be enough for everyone at present.
The TNBW Premium and TNBW Free groups are important. Everyone should probably join both groups. I'm not sure if non-paying users can join TNBW Premium. Premium is where we do most of our communicating with Sol and each other about the new site (bugs, enhancement requests, etc.). I don't use the Free group, so I'm not quite sure how it differs. I know that there are both free and paying members on the site, but I'm not sure how those two groups differ and how best to use them. I use Premium and Science Fiction exclusively. However, when you publish chapters, you can choose which groups to publish to (Premium, Free, YA, etc.). If you publish to the Free group, non-paying members can read your work if they so choose. I believe there's also a way to publish chapters/your work to be accessible to the whole the Internet. I don't use that, since I prefer not to expose my work for someone to copy/steal.
There is currently no place on the site like the old forums on the old site, where you could go into a specific forum and start a discussion about a particular topic. Each forum could have multiple different discussions going on at the same time. In order to do the equivalent here, you go to the group you think is best for your discussion, join the group so you can post to the group-specific forum, and post/read. Right now the site is rather quiet, hopefully just due to the holidays or people finding their way around. There is some overlap among groups, so you may have to join several to cover all your bases (e.g., several variations of young adult material, a separate fantasy group from science fiction, etc.). Like I said above, you can join ten groups, which seems to be enough for most.
I've read that Sol is going to bring back the old style forums to this new site, but that'll probably take time. In the meantime, Linda Lee created this group, Old Forums, to which you posted. It's a limited workaround. Read some of the other posts to this group to see what the limitations are. Although I'm a member of this group just to keep up to date on what people are posting, I don't use it for discussion threads. This group will probably go away once Sol implements the real old-style forums.
Some people have taken to creating groups for their specific pieces of work (e.g., a group for a specific book so they can get additional feedback). That's probably not a great idea, since we can only join ten groups each. One group per book would probably make the site a mess. The old site had a forum called Additional Feedback and there is a corresponding group in the new site for that that anyone can use for further discussions of someone's work. You can also get additional feedback about your work in the groups to which you post it. For example, if you post your work to the Science Fiction group, the forum there can be used to discuss your work (beyond the basic regular/inline reviews). In that group, you would get posts from people especially interested in science fiction.
There is also a Writing Tips & Advice group that I created intended to replicate the similarly named forum on the old site. Not much activity there yet. In that group you can find a how-to-write tutorial from Sol, and my discussion thread related to it.
I'm sure I've forgotten some things, so feel free to ask.
Happy New Year!