Transferring in to WP from Squarespace

Dear WP Community,

I’m quite new to developing websites. I used squarespace to accomplish what I needed to with my site to start with, mostly blog content. However, now that I’m growing I want to add functionality that WP.org offers with plug ins. I’ve been searching for hours and just need some help

What I am wondering is what is the best way to revamp a website that already exists by moving to WP?

After finding a new host, how can I transfer the domain to WP? Do I build the site on some other domain with wordpress and then just transfer the current domain? Does the old site stay live? I have no idea where to really start to be honest. Just looking for a branch to grab on to.

Thank you for any help!