How to launch WordPress site with template?

i started a wordpress site on my mac using mamp and imported a template. i finished it but i don’t know how to change the URL and launch it. on my wordpress admin login site, it doesn’t have a domain section like a normal wordpress site that you can only pick there templates. i have tried changing in the general settings area, the wordpress address and site address but then my wordpress site seems to go offline cause nothing will load. do you have to purchase a domain from somewhere else then change the urls? i am just confused about it. sorry it’s my first wordpress site.