Jules, if you'll start with the two tutorials that Harry T posted as Sticky threads
here, you'll find the first one (Book Creation Tutorials) suggests what formats your starting text should be in and then describes how to use BookDesigner to make an ebook. The second one (Tutorial - How to Create a MobiPocket ebook) tells you how to take a BookDesigner file into a MobiPocket ebook using Mobipocket Creator. If those don't answer your questions, ask again here. There are other places to find texts that you can use to create ebooks with besides Project Gutenberg. But if it's not in the public domain, then you won't be able to do so legally for others and so you won't be able to post them here.