Hello:-)

I'm preparing to make a survey using lime survey, I am using a Mac and have downloaded MAMP to run it. How ever, it's not clear to me how it works with getting the subdomain? I don't have a domain, so would I have to get a domain first? I don't want to run my survey on a website but send it out via an URL (via Facebook and mail).

I hope someone can help me, cause I'm a little lost here

You can't run Limesurvey without "a website". Your survey must be accessible on the web, which hopefully your MAMP based webserver on your Mac is not (this would be a serious security issue).

And: Yes, to have a subdomain you first need a domain. So even if you want the survey only be sent out via URL you need to point this URL somewhere, which would be your webserver. The local version on your notebook/desktop is for testing and developing the survey only. If you want someone to do the survey, they need either use your computer to input data or you need to publish it on a "website"