Let's go through your first chapter first.
...let me try that again.
Let's start at the very beginning. A very good place to start.
When asked about this new and exciting branch of cultural and popular history, Professor Monmouth seemed rather dismissive: "Castles in 8th century Scotland and Germany in 10th century Europe? Honestly, next you'll tell me they made Poland go into space and the Aztecs cross the Atlantic!"
This means that if the kingdom of the Rus existed, the king of it could claim that land as part of his kingdom because its supposed to be there. The Latin phrase means, essentially,
claim by right. Thus London is a de jure part of England.
What this means right now is that it would be easiest for you to aim to become king of the Rus by creating that kingdom, because you already own a bit of it. Unless you expand into some other region in a big way early on.
Good luck with that. The Rus generally do unite in my games and with what I've seen, but I have
never seen the Russian Empire come into being in CKII. The Rus generally attack Finland instead (which I guess is actually appropriate) whilst most other kingdoms that could make the empire get crushed, stay very small or expand elsewhere.
But I suppose therefore that is a challenge and grey unknown area for you to shoot towards!
Vassals not minding raised levies is going to be useful in the short term but to be honest I love feudalism a lot more in this game because I love my slowly centralising states. If you stick with Tribalism though, you are going to be at war. A lot. Basically, unless you do some very specific legal work or have literally one heir, your realm is going to get divided up upon your death between your children, usually into three or four big bits. Every single generation is going to have a large-ish war to get your holdings back together again, unless you want to abandon some parts of it to the rest of your family whilst you develop somewhere else.
Some like this, others don't. It is another decision you can make, go back on and make again as to whether you want tribalism or something a little more stable.
At this stage, you want women with either amazing genetic traits or good stewardship. A powerful marriage alliance is nice if you can get it (for most people, this means immediately marrying into the french royal family and letting West Francia do their fighting for a few decades) but if you want to build up your own realm and keep most of the land to yourself, you are going to need to either stick with tribalism, or choose some specific laws and/or get high stewardship with feudalism.
Um...well, in my experience it's possibly the one place in the British Isles that the Scots, the Irish and the Saxons all have a reasonable shot at conquering and thus I've seen quite a lot of warfare fought over the island when the AI is in control. Even when I was controlling England-bless me,
Camelot-the AI seemed quite interested in it. My vassal merchant republic was invaded there by the same duke
three times before I imprisoned and executed him and gave the republic the whole of northern Ireland instead. They then promptly started a war to get the island back

. So yeah...should be plain sailing.
Bribe everyone into positive figures right now and then figure out who is useful/threatening. Either do it with cash or titles or a mixture of both. Don't give them conquered land at this stage, you are way too small for that and tribalism is much more about personal control anyway.
Tribes get them at the start and you can summon more for wars using special events. You do need to use them though, otherwise they go berserk and start attacking you.
1000 gold is pretty good to start off with (for reference, go to the ledger and see how rich everyone else is. Bar massive empires and the Pope, no one tends to go above a few hundred at any one time due to expenses...and, you know, people like you stealing their money).
You have the unique opportunity to reform the religion in your own image if you want, so piety can be ridiculously powerful if you want to stay the course and not join the priesthood of Christ. Prestige is really important but you are a tribal chief seeking to conquer land, you aren't going to struggle for prestige if you win wars. And you should, at least a few.
Vassals, as I mentioned above, are not going to figure in a big way for a while with you (unless you get
a lot of land really fast or go feudal). Your vassals at the moment should just be lesser men, tribal chiefs and clerics (i.e. not even running one county but a subsection of one. In feudal systems they would be barons, mayors and bishops). What that means is that you shouldn't hit the vassal limit for a while because you don't need to give land away (you can run it yourself) and as a tribal civ you don't build that much infrastructure that needs to be managed by lower level vassals.
Generelly, the bottom tech in each column is the most important, as they affect armies as a whole, how fast you can build stuff and what laws you can pass. The other ones to invest in are sort of up to you, though generally castle and keep techs are good, and investing in whatever soldier type your civ produces lots of is obviously a good thing as well.
When this pops up, it is usually because a title is currently going to be inherited by another family other than the one that is holding it now, or the title is going to be inherited by someone that is a vassal to a different person than the current title holder (an example. Say you are the king of England and your vassal, the count of Winchester, gets a new heir. This heir, for whatever reason, is the vassal of the duke of York. Should the heir inherit, then Winchester remains in England but is a vassal under the Duke of York, not you).
However, it can also truly mean that the title is going to leave your realm entirely (this is much easier with bigger and more important tiles, unfortunately). So far example, the Duke of Normandy is a vassal of the king of England, but his heir is the King of France (this can happen. French intermarriages are fracking insane). If that heir inherits, England loses Normandy to France..and there may or may not be a war.
The pain of this alert is, it probably means nothing. BUT IT COULD MEAN a massive sea shift in the balance of power amongst you and your neighbours. So you always have to check.
You pick an ambition and then you either fulfil it or can change it once in a blue moon or so (every decade or so I think off the top of my head). Generally, people pick the groom and heir or get married ones first before picking unite a kingdom, unless they know they are going to get a kingdom within the first five years or so.
Getting someone into your realm voluntarily usually wont work unless you are the
de jure ruler of that place anyway. For example, Ireland is a favourite for starter players because you begin with one county (same as every other Irish count) and have to merely conquer half of these easy targets before declaring yourself king. The rest will declare fealty to you very easily if you ask them after that.
Well...three pages to go. I hope this helps in some way.