I'm fairly new to these topics myself, but here's what I've learned so far...
I turned my side hustle into a full-time LLC three years ago. I have a separate EIN for my LLC (it's basically a social security number to identify your business).
Zero plans for employees. In fact, that's my worst nightmare. I subcontract small things (a few thousand dollars here and there) but will never employ anyone other than myself. Whether you have employees or not is a consideration in what type of retirement plan you choose.
Last year, being frustrated that I couldn't contribute more to retirement and not knowing what to do with my extra money, I finally realized that SEP IRAs and solo 401s exist. Oops! Most of my freelancer friends are totally broke... let alone saving for retirement... so it simply wasn't on my rader.
I opened a SEP IRA because my accountant and business coach recommended it to me. They said I could contribute up to $50,000 (or whatever the limit is--it changes each year) and that should be plenty. Well, neither the accountant or the business coach are aware of the FIRE community. They're on the "retire at 62 cuz that's early, right?!" track.
Based on my 2017 net business income of ~$193,000, I think I'm able to contribute ~$36,000 to my SEP IRA. I've put in $10,000 so far and plan to put in more for the 2017 year this April when H&R Block finishes my taxes and I figure out the exact max.
$36,000 in a SEP IRA is great, but I think the individual 401k will be even better. I'm probably going to open one this year in 2018 and start contributing to it next year in 2019. My understanding is that I can contribute the $18,500 as an employee AND 20% or 25% or whatever as the employer. It should allow me to contribute around $50,000 total.
I'm not sure how to access either of these options prior to 59.5. There are probably tons of posts on this topic already. One of my 2018 to-do list items is to research this topic more.
Good luck on your learning journey! Hopefully my story helps you understand what others are considering.