This is a very broad subject, and I’m not much of a Poser user so I’ll probably address only some of the things you can do. I’m also trying to keep this as short as possible so I may not explain everything properly.
1) Customise the lay-out: you can have Rooms if that is what you like. Go to Window->Workspace->Select Layout and pick either Hollywood Boulevard or Self Serve. You can then click at the top of each “Activity tab” to change “room”. City Limits is the Layout with the greatest number of icons, though, but it doesn’t have activity tabs.
You can also customise which tab goes where, simply click on its name and drag it into the view-port to un-dock it (you can also move it to the other tab directly). To spawn a tab you have closed by mistake or that isn’t available by default in the layout go to Window->Panes (Tabs)->and select one. Puppeteer and Powerpose may be two tabs you may like to experiment with.
Finally, pretty much everything can be customised with Window->Workspace->Customize (F3).
2) Content: sigh. I think I’ll leave this for others. I’ll just say that only recent products (Genesis generation for the most part) will show up in Smart Content. However all of your installed content will show up in the Content Library tab. In this tab right click on “Poser Formats” and select Add a Runtime Directory… to make DS aware of your Poser runtimes! Above Poser Formats is DAZ Studio Formats which hosts (almost) all native DS Content. To load a Genesis figure go to DAZ Studio Formats-><DAZ Studio Content Folder, which is “My Library” by default>->People->Genesis, you can either double click on it or drag it in the viewport. Things are organised a bit differently than in a Poser Runtime, and sometimes you have to search for stuff under different sections (e.g. the DS preset for Marie Elite is somewhere, but other Elites tend to land elsewhere, for whatever reason). There is also a Categories which lets you organise your content as you like, but I’ll leave it for those who use it. You can multiselect (Shift or Control as in Windows) several objects/figures/props/materials and drag them in the viewport to load them all. Pressing Alt while dragging will generate a yellow and red tool to load a figure/prop in a specific place instead of <0,0,0>.
3) Once things are loaded, they show up in the Scene tab so you can select them. There are several different tools for interacting with your scene items that appear just under the Activity tabs. There are tools for “node” selection (everything that appears in the scene tab is a node, be it the figure root or just a body-part; camera, etc.) and posing and for surface selection. You can create new lights and cameras. By default there is a light parented to the camera, which is unselectable and will be hid if you add a light. You can look through those cameras and lights by clicking on “Perspective View” and selecting the light or camera instead. You must select a camera in there to render. If you drag a clothing figure on a figure it should “Fit to” (autoconform) automatically. If it doesn’t you can go to the Parameters tab, click “Fit To” and the figure it should be fitted. If it’s a V4/M4/K4 clothing that you added on Genesis this will also trigger the infamous Autofit. 
Hair props (and all other props) can be Parented to a figure head, or other appropriate part. Go to Scene tab after having loaded them, expand the figure tree until you see the Head and drag the hair prop to it.
4) Cameras tab and Lights tab work on only one of those at a time. If you want to adjust several Lights (colour, intensity, etc.) at once multiselect them in the Scene tab and make the changes in the Parameters tab (in general most of the interesting stuff goes through the Parameters tab, rather than the more specialised tabs; another tab to keep an eye on is the Tool tab, which gives plenty of options depending on which selection tool you have active).
5) Nulls for moving or rotating stuff all at once. Select all the things you wan to be parented to the Null and go to Create->New Null. In the new window select “Target New Null with selected item(s)”. You can then rotate the null (in Parameters tab or by hand) to rotate all the parented items.
6) Right click everywhere. Plenty of options are available usually by right clicking. Most stuff is now available also in the Edit menu, but it’s a fairly recent addition.
7) Options buttons. At the top inner (towards the viewport, but if the tab names are horizontal it’s always on the top right corner) corner of all tabs is a little triangle followed by lines, click on it to reveal lots and lots of options.
8) Add an Author name, if you haven’t already, that will “unlock”, I guess, the Save option. Otherwise it acts as a Save As function (this is to avoid overwriting scenes that aren’t identified as made by you).
That’s what I can think of for now.
