The answer partially depends on whether you have a managed or referenced library. Since you're totally new to Aperture, I'm assuming you have a managed library since that's the default.
FYI: Managed means Aperture handles image files and places them inside the library file. Referenced means you manage the image files and Aperture only points to (references) them.
For a managed library, select an image you want to delete. Type command-delete. (That's the reverse delete key next to the "=" key). (You can also right-click an image and choose Delete Version) That will remove the images from their Project and place in the Aperture Trash. They're still taking up space in the Trash. If you really want them gone for good, right-click the Trash in the library pane and select Empty Trash. Poof.
For a reference library, I forgot the exact details since I don't use this type of library, but you'll be asked (or can check a box) to delete the original file from the disk.
You might also want to take a look at this user tip Deleting version also deleted the master.
As Thomas wrote for referenced originals you will be given the choice to delete the originals from your disk when you empty the Aperture trash.
And in all cases to finally remove the files from your disk you must empty the system trash as well. The deleted are placed in the system trash after you empty Aperture's trash.