First of all, welcome! You’ve already made great progress getting openHAB installed and running to the point you’re at.
To fully appreciate openHAB and use things like sitemaps and rules, you’ll definitely need more items in your configuration. You may have already read this in the tutorials, but from a very high level, these are the building blocks of openHAB:
Things - Hardware (or software) components you want your home automation system to interact with. For example, a Samsung TV or an HVAC thermostat could be a thing, or the Sun could be a thing. Things have predefined channels to access their controls or data (i.e. Samsung TV thing will have a volume channel, or the Sun thing will have a position channel). You can find the things in your HA system via PaperUI, but read on first.
Bindings - These are the addons (plugins) to the base openHAB system, which interface with the things and expose their channels to you (i.e. Samsung binding will find Samsung TVs and provide access to its volume, mute, power, etc. channels, and the Astro binding will provide access to the Sun thing’s position, angle, season, etc… channels. You can use PaperUI to install bindings, but my preference is to use the addons.cfg file in your openhab-conf/services folder.
Items are variables that connect to the channels of your things and are used in the next few blocks (sitemaps, widgets and rules). These should definitely be defined in *.items files within your openhab-conf/items folder. You can start with a single file, but once your system gets larger, you want to split them up (i.e. use different files to store items for LEDs, lights, HVAC, sensors, etc…). Items are generally each tied to a single channel of a thing (e.g. you’ll have a Number item that connects to the volume channel of your Samsung TV thing and store the current volume level)
Sitemaps organize your items into user-friendly UIs (Basic UI or Classic UI), usually on a functional or architectural grouping (e.g. you’ll have a sitemap that breaks your house into floors, and puts all second floor items in a group that shows when you click on the second floor sitemap frame.
Widgets are a special type of UI building block, used by the HABpanel UI (check out my video here and this blog post from @dcabanero to get an intro on Habpanel). I can honestly say if I was starting openHAB now, I’d create all the items I wanted to control first and go set up a Habpanel UI - it’s the quickest way to a working user interface and easier to learn than sitemap syntax.
Rules are the real point of all of the above - they tie everything together and are what makes your home automation system really automated - that is to say, they perform logic and actions/commands on your items based on the items’ values or other triggers. For example, you could set up a rule that fires when the Motion Item (which is tied to the motion channel on your Zwave motion sensor Thing) receives an update to “ON”. The rule could check the state of the isNight item (which is tied to the night channel of your Astro binding’s sun Thing) and send a command to the HallwayLight Item (which is tied to the power channel of the Phillips Hue binding’s hallway light Thing). You can look at my tutorial to see how I tied Google Home, Phillips Hue and other bindings and used rules to set up entire scenes for my items. Rules can get very complex very quickly, but there’s no better community than this forum to post your attempts and get a quick, fully working rule figured out (usually by @rlkoshak who I think is the most experienced rule expert here 
).
Anyway, this is the basic description of the building blocks, and I used bold text to make you familiar with the interactions between each block and get you thinking/talking in those terms.
As a starting openHab’er, I’d recommend you go into PaperUI and check out all of the bindings you have available. Install any you think you might use (you’ll probably want to install the Astro binding in any case), install the additional UIs (like HABpanel), and start playing with the Demo items files, trying to tie them to the channels on the things you find in your setup (i.e. your phone, network devices, etc…). Also, check out the Tutorials & Examples section of the forum, as well as the new website’s Showcase & How-To’s section, to see some of the fully-working examples others have posted.
Also, check out the VSCode forum topic to set up your extension (it’s become the de facto editor for openHAB configuration files, and it definitely helps with the auto-complete and code validation features).
Enjoy and let us know what else you need to get started!