Hello @Daniel_Hermann1 and welcome to openHAB! 
Let me try to answer all your questions. I have one favor to ask in return. Please re-define a few questions that you thing are missing in the FAQs:
You can choose between Paper UI, file based configuration or a mixture of both.
Hereās how I use openHAB 2 currently: PaperUI is a great help with basic system settings and add-on management. For things and items definition I am relying on configuration files. You can also use Paper UI to auto-discover things and items - I just like to have total control over what is defined how. You can go both ways really.
Your decision should also be motivated by the bindings you are using. I am using the homematic binding for example and while I build my configuration files with (a lot!) of items myself, I could just as well relied on Paper UI. Remember to check out the Binding documentation for the āFull Exampleā, e.g. http://docs.openhab.org/addons/bindings/hue/readme.html#full-example
In the beginning you should look out for Native Bindings. Later you can also look into Legacy 1.x bindings.
Iām not sure what you are talking about. There is a button for that, no?
Check my setup here, here and here.
If possible in your device. If you need to process an Item beyond itās basic properties, use a rule and a virtual item (this is only one possibility): Design Pattern: Unbound Item (aka Virtual Item)
Canāt answer for sure. Why do you need tags? Itās a quite new conceptā¦
Yes, thatās exactly what āgroupsā are forā¦ http://docs.openhab.org/configuration/items.html#groups
Rules in openHAB are best build in rules files, these are text based script files which are maybe not as easy to write (way easier than FHEM scripts) but extremely powerful. Look out for all kinds of examples in the forum. e.g.