So, say that I wanted to start a game of warband, where and how would I start.
Don't go straight into getting vassalage. Build up capital first. That little village you get is not, in the least, going to be a source of income for you. You almost always get some desolate village on the border that is constantly being raided, and trying to defend it will get you killed, either by attrition or by attracting an entire army. Instead, spend the first two or three months as a small warband. Build up an elite cadre of troops, do task for the guild masters and lords, and buy up profitable businesses. Get one in each faction first, and then, if you know which faction you want to join, more in there too. That way you don't have to worry about running out of money, even if your homeland is entirely razed and several factions are at war with you.
If you find that the pace of getting money is too slow, you have two options. First is to take up actual trading. Go from town to town, and check out prices. Not sure if, in vanilla, you get the option to find out which towns to trade to at a profit, so you might have to do that manually. Make sure to note that as a town gets less of an item, it gets more expensive, and as it gets more, they get cheaper. So if you buy an item, the next item will be slightly more expensive at that town, while if you sell one, you will get slightly less money for the next. As you travel from town to town, see if you can get a job delivering ale or guarding a caravan. Don't bother with cattle. Cattle are annoying as fuck. If trading is still too slow, become a mercenary. The wage won't give you a profit, but it will cover most of the cost of your troops. The true way you'll make money is raiding. Find a bunch of villages, preferably Khergit , or a faction/village you don't plan on getting troops from, and loot them. You can often get very large amounts of butter, which you can sell for an average of 2000 denar altogether. Put that money back into buying businesses.
Business will take a couple of months to turn a profit normally, but eventually they will, and if you end up losing a battle and all your money is taken, with your troops killed, you aren't going to just be starting over.
Is there any particular troop composition that you want to play? If not, use the golden trio of Rhodok sharpshooters, Swadian knights/ Sarranid Marmalukes, and Nord Huscarls. Sharpshooters will murder the shit out of absolutely everything at range, and are the god of sieges, both defense and attack. Knights and Marmalukes are basically exactly the same, except one is slightly faster and the other has a little more armor, IIRC. A good twenty of them is perfect for destroying lines of archers and enemy cavalry charges, and if needed can go toe to toe with most infantry. Incredibly expensive, so you might want to hold off on them for a while. Huscarls are very heavily armored, will kill any unit they close in on, and as a bonus they have a bunch of throwing weapons that can decimate an enemy infantry charge at range. If you have to, you can substitute Swadian sharpshooters for the Rhodok ones, as they are only slightly inferior, I think. Other troop combinations are viable, but they require more advanced tactics, or have big weaknesses. The Vaegir top tier infantry unit can actually kill Huscarls if they get into melee, but will be murdered long before then because they don't have shields. Same goes for their archers, who can kill sharpshooters but don't have any kind of tower shield, and their arrows don't have the damage necessary to stop a charge in its tracks. Khergits can kill many armies, but are shit at sieges and have no good infantry units. Rhodok and Swadian infantry would be good, but pike infanty in Warband aren't really as useful as they should be.
The top tier infantry are rather expensive, so don't just upgrade them the moment they can level up. Try to keep your weekly upkeep at about the level you get from your businesses.