If the photos are in different projects, then create an "album" with a suitable name and drag the images to that album. An album does not contain copies of your images, just references to them. Albums consume only a very small amount of disk space, so don't worry about creating clutter.
When you have finished dropping images into the album, you can select any or all of them from there.
If you find that an album is no longer of use to you, delete it. Deleting an album doesn't delete the images, just the references.
Again, the Aperture User Guide is really good. Open it in Preview and do searches.
Other resources are the Aperture discussion group over on Apple's website, and the fabulous training book by Dion Scoppettuolo - "Apple Pro Training Series: Aperture 3" which is available as an iBook from Apple or hardcover from Amazon. The electronic version is more useful IMHO, since you can search it. The training is hands on with downloaded files and you follow along on your Mac.
You'll find yourself saying "Wow, Aperture can do THAT!!? Incredible!!!"
Meanwhile, I suggest you get connected to the Apple Discussions for Aperture and check out the postings there, and if you can't find that your questions have already been asked and answered, the people there are deeply knowledgeable and helpful.