All very easy - the trick is to remember that terms like "project", "album", and "stack" are very arbitrary.
Projects are simply the place where the images are stored - I think of them like rolls of film and tend to create a new one every time I unload a card. The only limit is that you should not have over 10,000 images in one Project. So think of "Projects" as rolls, boxes, events, envelopes - whatever you consider your lowest organizational element. Mine are simply named by date and a hint e.g., 201210 - Trip to Zoo. The real retrieval data is found in the Albums above this. I use a geographic structure - Continent>Country. I don't use dates as Aperture creates Smart Albums by date automatically, so there is no need for me to reproduce this structure.
Stacks are a simply a way to group photos, usually variations of one image. For example, if you are shooting multiple frames per second, you might group each burst of images as one stack. This is simply to avoid cluttering your browser and is not really an oganizational element.
Albums will do what you want. There are two types, Smart and "dumb." Smart albums are really database queries that run automatically. Aperture come pre equipped with several, including most recent years. But you can get very clever with them. For example, I have moved to a mostly referenced master setup, so I have a Smart Album that finds all of my Managed Masters so that I can periodically move them to the referenced drive. Keywords can be very, very powerful here.
You can also simply drag an drop images into Albums. Remember that images in an Album are merely database pointers. That is, they appear in the the Album, like a copy, but any action you take actually affects the original image. (To make real copies, you must create a new "Version.")
Hope this helps.