Getting started in a faction- Depending on your choices in the character creation. You can choose to be a Commoner or a Noble. The choices change the dialog initial relationship, and how Lords speak to you. If you choose to be a Noble you receive a banner and 100 renown right off the bat. Also as a Noble Lords you speak to will see you as a faction-less lord. As a Commoner you get remarks on your common birth when you speak to lords some good some bad just as Noble. You have to keep an eye on these things. Knowing their minds and attitudes is the key to the door of Calradia.
Now to the point. You make your character, and You need at least 150 renown or a high reputation with a King or a mix I believe, to become a vassal. My understanding is that every ‘point’ of relationship you have with a king reduces the amount of renown you need by one ‘point’. That is easy to get if you are a Noble considering you start out with 100. If you are a good player or you have a low difficulty level you can solo all types of bandits or deserters for a quick fix to get your renown up enough to be accepted by any King. Tournament's maybe two of them and your set as well.
Well let's say your set, all you need to do is talk to the King you desire and give homage to him if he accepts you. There is also a chance that you can be invited to be there vassal if your a famous man or women. Once your accepted if you're a Commoner you will receive your very own choice of a banner. You will also get a fief of a town of the King's choice. Hint, he will always choose the least profitable fief in the kingdom. Even worse, whoever owned it before you will have a significant negative relationship with you, you upstart.
If you do not like your current faction you can leave. Talk to the King select the choice to leave, and when that's all said and done you can go. The negative effect is you will lose 20 reputation with the King and loose all fiefs you may of acquired.
Creating your own faction- This path is one of the hardest path's in the game along side of choosing to rebel against a King. In order to start this I recommend you to be around level 25+ and have a high level leadership, alongside having a stable companion group, and roughly 80,000+ denars, and right to rule is helpful. These things I said take a while to obtain without cheating. So join a faction for a while maybe even the faction you are going to attack and get your reputation up with the lords there, and the King.
Now you need to target a City (recommended) or a castle. The reason I recommend a city is because if you get a castle first you cannot give it away as a fief to a companion or Lord if you get a city later. The first fief you get is the foundation of your new empire. Note that I believe you CAN give it away if you first move your court to another castle/city. That said, I’d still recommend a city because since when were great empires ruled from a border keep? On a more practical note, castles come with only one village while cities come with at least two, and usually more.
How you start to do this is to attack the enemy's assets farmer, caravans, raid a village or attack a lord. This will give you a negative relations with that faction and allow you to besiege the fief. Now the reason in the beginning I said you need to be ready is when you finally conquer the fief and it's yours, and all is well, it will be priority one to take back. The faction you attacked will come in a massive force unless circumstances are really bad for them. They will most likely siege you within a week of you taking their fief you will also at that point have a -40 relations or more. He’s not joking here – you can have a thousand or more troops knocking politely at the gate within a week. For this reason I STRONGLY recommend picking a faction who is already at war to be your target, bringing around 70+ elite troops (huscarls are ideal) and as soon as you have the town leaving the elites and frantically raising as many troops as you can from the villages. With companions with good training skills and some luck you might have as many as 150 third tier troops to help out. Don’t forget to check the taverns for mercenaries – 6 hired blades can make all the difference sometimes!
Ok, I am going to head back to when you take the fief. You will be given a choice to pick a minister/ambassador. It can be either a companion or if you are married, you can choose your wife if you are male. If you want none of the above because you don't want to waste your one of your companions or you are not married yet or your female, you can appoint a temporary citizen from your castle/city. Who is utterly useless. I’d recommend getting a companion you don’t like or won’t use specifically for the purpose if you are playing a female or unmarried character.
Now at the hard part. In order for you to hold your faction in place you are going to need soldiers. I would put roughly 300 in a city and make some of them fodder for money reasons and make some elites as a backbone for your defenses. I also would have a cavalry section to chase away or fight other lords that attempt to raid your towns. This will be costly so don't give away initial towns to Lords right away you may need all the fiefs you can get to pay for your garrison. I find about 250 in a city, of which 50 or so are tier 4-5, 100 are tier 2-3 archers and the rest are fodder/militia strikes the balance between cost and the ability to actually fight. Normally I have my ‘capital’ more heavily defended than this, as I use it as a storage space for elite troops to top up my own forces when I take casualties in war – it can be a good idea to have an entire ‘spare’ party in case you lose everyone... Around a 100-150 in a castle is plenty, mainly low level.
Another option to start your own faction is to capture a castle or city while you are a sworn vassal of one of the other factions and ask for it to be given to you. If the king agrees, great, garrison it and go and do it again. If he refuses you get the option to rebel and keep all of the castles and cities you own, along with their garrisons. Note that if you don’t own the city/castle which goes along with one of your villages you will lose it. This is a much, much easier way of doing it, as you can stack your castle with several hundred elite troops and simply defend against the hordes that come your way, but if you wanted easy, you’d just use cheats, right?
Start a rebellion- I will not lie this gets real difficult. The first thing you do is join the faction you want to rebel against. Get a castle ,throw festivals, station a "small" garrison, and make money lots of it. Along with reputation will all the lords there. When you have an amazing party size roughly 120+ some right to rule,your companions leveled, and a hefty wallet to sustain troop size for a long time you’re ready. Talk to the Claimant you wish to have join you, and he or she should accept you if you’re powerful enough. (Claimants can be found by travelers once you leave your faction) He or she will join your party as a companion, and will have data on the lands and how many lords the faction has. You will also immediately get -40 reputation bonus. At this point it's started you did it if you followed my advise here. Head to the territory you are rebelling and single out the lords there. Talk to them and persuade them to join you. Do this to whoever you can and fast because once they join you, they will retain all of their fiefs (if they have a castle or city). Make sure you get yourself a castle or city some garrison and have fun.
Alternately, you can do this the even harder way by not joining the faction first and doing it without any lands to support you. I recommend it as a challenge, because without having any relationships with the lords beforehand it’s actually very difficult.