I think you'll find this short introduction to Aperture helpful. (I wrote it in response to another poster. Discussions formatting has since changed. In that post, the text between plus signs -- "+" -- should be understood as being italicized.) I also strongly recommend reading and understanding the first seven chapters of the Users Manual before doing anything with any important images in Aperture. The User Manual is available on-line, as well as from the Help menu. Those chapters cover the lay of the land of this new continent called "Aperture". Until you know them, you will trip on ropy vines and fall into leafy crevasses. Tripping and falling is frustrating.
Keep in mind that Aperture is an image database, not a file manager (though it does some of the latter). Many users find it difficult to accept the novelty of having this image management primary (I had the same difficulty). The sooner you come to terms with this image-centered workflow, to sooner you will be able to take full advantage of what Aperture is and how it works.
Aperture won't lose your files. Your bargain with it is, in order to use Aperture to manage your images, you are going to let Aperture manage your image files.
There are many fine posts in the forum re: Managed vs. Referenced image Masters.
When you are ready to delve deeper, take a look at this discussion. RWBoyer's answer is full of insight gained from years of use, but as he says, "It really is very very flexible but you may just have to get some experience on your own before you see the light".