What is Cubase? I mean, I know I can record and arrange Music with it, but can I use it, together with a MIDI keyboard
to “build” a synth like Yamaha MOTIF style? Do I need a special kind of audio adapter, internal or external to get better sound quality? Where can I learn more about this? Any good guides (youtybe, Vimeo etc)?
Thanks,
Hi, welcome to the steinberg forums 
Cubase does an awful lot of things. Yes, recording and arranging music are some of the main features.
Cubase itself is not a synthesizer, but it is a VST host which means it can host VST plugins. Cubase comes with a selection of VST synthesizers, instruments and effects that you can indeed control with a midi keyboard, and there are a gazzilion third party VST plugins available online, ranging from free to ridiculously expensive ones.
Whether you need a new audio interface depends a bit on what you have now and what you want to do with Cubase. If you want to play virtual instruments in realtime with your midi keyboard (without noticable latency), the built in soundcard of most PC’s isn’t great, but a good starter audio interface doesn’t have to cost more than €100,- (depending on your requirements of course, if you want to record an orchestra you’ll need something else 
)
There are tons of videos online on specific features of Cubase. If you are completely new to digital audio workstations (DAWs) then no matter where you start, it’ll be quite daunting at first . If you search youtube for ‘cubase basics’ you’ll find plenty of results. Add specific topics to your search query if you want something particular.
Final bit of advice, if you’re looking at tutorials, check which version of Cubase they’re for. Cubase has been around for a long time now, and although the features of older versions are still available in newer versions, they might look or work differently. Before Cubase 7, the mixer window looked very different for instance.
We’ll be happy to help if you have more questions, but please be a bit more specific because the question ‘Hi I’m new, what do I need to know’ is a bit big to answer in 1 post
Don’t think I am trying to put you off here, but DAWs like Cubase are probably the most complex pieces of software you will come across that can run on your PC.
This is because along with multiple audio streams, they also host as many FX and software instruments – from Steinberg and third-party – that you can load in them, and all running near real-time.
Now, that does not mean that you will have to know how to operate everything before you can do anything, but just that you will get the most out of it in the long run if you undertake to learn more about its capabilities as you go along.
I would recommend that you get the Operator Manual, and read the first paragraph of every chapter. That will give you a good idea of the breadth of its capabilities.
Then read the first paragraph under every heading and subheading. That will give you an idea of the depth of its capabilities.
Now while these two activities will not take all that long, and in no way enable you to do any one thing, the knowledge will be sitting in your brain for that time when you need to do something, and you will have a good idea of what is the best way to do it. Then you can go to get the drill on the how-to just in time to actually use it.
Don’t avoid this stuff, otherwise you may just end up doing things in an inefficient way, when there are plenty of tools to do it efficiently.