Well, you don't give us any info about what you have tried, no project settings, no render settings or codec choices, but here is an overview that might help.
HD and quality are completely different things. HD involves the number of pixels in your footage, that's it. Quality involves the detail and color data contained in your footage.
To retain the correct HD resolution, you have to make sure you create the project at the correct resolution. Every time you start a project, you will see a project settings dialog:
You need to make sure that you use the same height and width as the footage you are going to import, as the size you select, or enter, here is the size that will be used for the final footage you export.
As far as quality, that all comes down to the codec you select when you compress your footage. Every time you render, you will see a window where you select a codec. As you mention being on a Mac, this is the window you will see:
In order to retain maximum quality, you will need to select a high quality intermediate codec. Excellent options in your case would be ProRes, Apple Intermediate, or Animation.
The aspect can be changed with a click in FCP, if necessary. Its part of the info in every clip. Hopefully this is helpful, its the best I can do without any specific info on your case.