I see that a lot of people like Scrivener better than yWriter. I know Scrivener is better, but my dad refuses to buy it, and I don't have a credit card. So, what does make Scrivener better than yWriter5?
If you're on a Mac, you can't use yWriter. :) Beyond that, from the screenshots it looks like yWriter only allows you to break into chapters, while Scrivener lets you have files in folders with no restraints on organization, as deep as you like, plus photos and audio files.
Scrivener has many tools, like Word Count Goals, frequency analysis, a name generator, a compiler, and the ability to do snapshots of your work so you can roll back to a previous draft. It's simply a more expansive, powerful program.
If you just need to get underway (and you're on a compatible platform), start with yWriter and save your money until you can get Scrivener. You can write in Notepad if you need to; you don't have to have Word.
Scrivener is now also available for iOS (the $19.99 app which is compatible with both Mac and Win desktop applications and allows you to sync the files over Dropbox), the features are subset, but still very useful.