Well, there's an option to pledge yourself as a vassal, though I've never tried it. When you're in your own faction you aren't part of anyone else's if that's what you're asking. However, you can join another faction. I just didn't want to.
As far as enemies were concerned I just made sure I had made friends with lots of people beforehand, both in the Kingdom of Sweden where I started and also with everyone I fought (if you release people after beating them your relationship with them improves). Then when I rebelled I tried to convince as many lords to join me as I could, while also having a nice army of my own. Most of the world is too far away to care, it was only some factions in my immidiate area that were a problem.
When you become king of your own faction, you appoint one of your followers to be part of your court. When you talk to them they can send emissaries (another follower, make sure they have good persuasion)
to other factions to make peace or trade agreements. Once you've staked out a sizable kingdom for yourself the other leaders start taking you seriously.
I started in Sweden as a mercenary, joined the army, made some money that way, then became a vassal. From there I captured some places from the faction they were at war with (I forget who it was , it was like Novgorod or something I think. They're east of Sweden at the start.) but when they weren't giving me any fiefs I decided to rebel and take what I fought for. I had to fight off Sweden and Novgorod for a bit until they decided that they were losing more than I was and asked for peace. I've since conquered Hungary and Novgorod, part of the Teutonic Order, and am readying to finish of Sweden. The important part is using the marshalship correctly to channel your armies where you need them. And make lots of friends everywhere. The fighting when you first start out get you lots of right to rule, after you have ~50-60 people take you seriously.
Apologies for spelling, I'm on mobile.