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This might be a good place to start. Watch the video and then try to do the same thing in game. It's a good way to learn basic mechanics and such. Then, if you want, you can move onto some of the Let's Plays on YouTube.

Have fun!
 
The main mechanics are pretty simple so you should be able to grasp them quickly enough.

For a good starting position for first campaign I'd recommend Shimazu. They got a nice position in Kyushu, and you can work your way through there, while the big boys are fighting each other at Central Japan.
 
So what rank is the best to start as for the first couple of games? Coming from Crusader Kings II we had Noob Island (Ireland) as independent counts, but it seems like Kokujin are extremely limited in what they can do (and of course, no independent kokujin really).

And is there a guide that illustrates the differences between Kokujin/Daimyo/Clan Leader?
 
Starting as a kokujin in Sengoku is very hard. As some has already suggested above it's better to start as a clan either down in the south or up in the north where you have fewer enemies around you.
 
I just bought the game yesterday. I have played the demo before and it wasn't too diffcult. I'm planning to play my first serious game as Oda family (I managed to find where Oda Nobunaga's ancestor is). I might try as some bigger famous family, but I don't know Japanese geography well enough to know where ancestral lands of those families were located and finding dynasties which became famous later is quite difficult in earlier scenario, because there are so many to select from.
 
I just bought the game yesterday. I have played the demo before and it wasn't too diffcult. I'm planning to play my first serious game as Oda family (I managed to find where Oda Nobunaga's ancestor is). I might try as some bigger famous family, but I don't know Japanese geography well enough to know where ancestral lands of those families were located and finding dynasties which became famous later is quite difficult in earlier scenario, because there are so many to select from.

I wouldn't recommend such a start for your first game but looking up a character that later became influential and start as him when he's still a kokujin is a fun challenge. We even included some famous historical characters as courtiers just so that they are around. That way you can start as their overlord and give them a title and then switch to playing them.
 
Starting as a kokujin in Sengoku is very hard. As some has already suggested above it's better to start as a clan either down in the south or up in the north where you have fewer enemies around you.

I'll be honest, I'm not sure I even know what I would need to do to break away from my daimyo when starting as kokujin - I've never successfully petitioned for a daimyo title from my lord, even with max 5 kori I have nowhere near enough power to make a clean break from my lord, can't recruit levies, not enough money to recruit enough ronin...

Outside of a lucky chance - is there even a consistent mechanism in the game to serve as representation of 下克上?
 
I'll be honest, I'm not sure I even know what I would need to do to break away from my daimyo when starting as kokujin - I've never successfully petitioned for a daimyo title from my lord, even with max 5 kori I have nowhere near enough power to make a clean break from my lord, can't recruit levies, not enough money to recruit enough ronin...

Outside of a lucky chance - is there even a consistent mechanism in the game to serve as representation of 下克上?

My humble request is that you'll explain what you are talking about. I don't understand Japanese characters, however I know some feudal terminology and I might understand what you're talking about if the terms are written with Latin alphabet.
 
My humble request is that you'll explain what you are talking about. I don't understand Japanese characters, however I know some feudal terminology and I might understand what you're talking about if the terms are written with Latin alphabet.

Apologies, I meant gekokujo (げこくじょう) - "retainer supplants his lord" phenomenon; 下 (shita = below, under), 克 (katsu = to prevail), 上 (jo = above, over).

Gekokujo was a phenomenon popping up all over the place in Japan towards the end of 15th century; many great families were "betrayed" by their retainers - Shoni by the Ryuzoji, Shiba by the Oda, the Asakura and other clans, Toki by the Saito, Akamatsu by Bessho and Urakami (in turn supplanted by the Ukita).

Hope this makes sense bud.
 
Apologies, I meant gekokujo (げこくじょう) - "retainer supplants his lord" phenomenon; 下 (shita = below, under), 克 (katsu = to prevail), 上 (jo = above, over).

Gekokujo was a phenomenon popping up all over the place in Japan towards the end of 15th century; many great families were "betrayed" by their retainers - Shoni by the Ryuzoji, Shiba by the Oda, the Asakura and other clans, Toki by the Saito, Akamatsu by Bessho and Urakami (in turn supplanted by the Ukita).

Hope this makes sense bud.

I know what gekokujō means, but I'm an ignorant gaijin who doesn't understand other writing systems than Latin alphabet and Germanic runes.

In my first game one of my kokujins demanded that I hand over to him the clan leadership and 3 other vassals supported him (none of them had daimyo title). I had support of only one vassal. I barely survived that attack. I haven't tried as kokujin yet, but can't you plot in similar manner?
 
I know what gekokujō means, but I'm an ignorant gaijin who doesn't understand other writing systems than Latin alphabet and Germanic runes.

Okies, sorry if the post came across as patronising, it was definitely not the intent.

In my first game one of my kokujins demanded that I hand over to him the clan leadership and 3 other vassals supported him (none of them had daimyo title). I had support of only one vassal. I barely survived that attack. I haven't tried as kokujin yet, but can't you plot in similar manner?

You can break away from the clan and form your own clan (if your honour > clan leader's), you have events popping up where you can petition clan head for another kokujin title or for a daimyo title; you can certainly plot but I don't think I ever managed to achieve anything this way... But you know what? You've actually inspired me to fire up Sengoku when I get home and check this out. Thanks :)