Just to add a little to what Nermander said above, there are many DTP tutorials on the web - and some of them are very good - but you need to find one that caters for the type of document you want to make.
For instance, a tutorial showing how to put a newsletter together isn't going to tell you much about how to create a book; and a tutorial about making a fashion magazine isn't going to be much use to you if you're making a company report. The possible layouts and design considerations for each are very different and the design for one will look wrong if used for another purpose.
A good way of getting to know your way around software like Scribus is to try and replicate what other people have already produced and then experiment.
First, have a look at documents that have already been created - newspapers, magazines, books, etc. - that are like what you want to make.
Then sketch out - on a piece of paper - the general layout of the page(s) in one of those documents and then transfer that layout to Scribus. I.e. Create text frames where the text on the original was, the same for images etc. Don't get too bogged down in the details of being exact. What you're trying to get is the general idea.
Then add some sample text to each text frame, and random images to the image frames. At this point you don't care about the content, you're just looking at how you manipulate things in Scribus.
Then start moving things around and note how the changes you've made alter the overall look of the page.
You can then start to look at using multiple pages. Add new pages and create different layouts. Move stuff around. Change the images to other random images to see how image "shapes" change things. What you create at this stage doesn't matter because you'll probably just trash it in the end. What's important is that you are experimenting with the software.
If you make a mistake, who cares? What does it matter if you accidentally delete something or it doesn't look right? It's just random images and sample text. You can't do anything wrong or break anything because the final result is pointless anyway. You wouldn't expect to pick up a pencil for the first time with the expectation that what you draw will be shown in a gallery and it should be the same with your Scribus work.
Play around with it. Make as much mess as you like. Change fonts, column settings, image effects. Create shapes and edit them with the node editor to see what happens. Drag things over other things and see how the text flow options work. Remember what happens when you change things and try and figure out what you can do with it.
Get used to being comfortable with Scribus before you make something that is important. If you're worrying about every little detail from the start then you'll probably be put off from trying different things and the whole process will be unsatisfying.
My point - eventually! - is that you should get used to using the software on stuff that really doesn't matter before you start worrying about getting it right on the stuff that does matter. Enjoy using Scribus and your documents will get better and better over time.
P.S. In general, unless you're getting your documents professionally printed on paper that will be folded and cut to make the final output, you really don't need to know about stuff like impositioning. (And even then your printshop will probably do that for you if you ask them nicely.) Basically, don't worry about it until you know you need to think about it.