WIth Xenforo cloud, the Xenforo team will do all of the work setting it up from scratch and maintaining it (the technical side, not the community) for you, you don't need your own servers or such like.
Depending on what you want it to look like you may then want to pay a developer to create a style and personalise the appearance for you (we use @Russ from PixelExit who is excellent but there are others if you look)
Assuming someone else sets up the basics, your main work initially is going to be deciding on what kind of a forum structure you want. I always think the simpler the better especially these days. But have a look round at various existing forums to see what they have done re: categories/subforums etc
I didn't know how to start either. I opened a support ticket to help me install it in the server. And the server also helped with that as well.
Once it's installed you start adding nodes. The nodes are the lists of different category topics. You could post about that when you get there for assistance. Also in the Admin panel there are various style settings to change colour etc. Or if you get one of Pixel Exit's styles that is all done for you (and adds various extras as well that refine things a bit if you want further refinements). And they are very helpful.
But if you mean start from scratch altogether then to start with you need
a) a server plan set up
b) an email package set up (either via the server or elsewhere)
c) a domain name set up and linked to the server (if you buy the domain name through the server that will be automatic or they'll tell you how to link it).