Really, I don't know how to sustain and train a large army, really. I can't make a kingdom without conquering, and without an army, I have no hope.
It's not that hard, but it really depends on your strategy.
I suggest NOT joining/starting a faction at all until you can gain ~10,000 denars. Tax money usually pales in comparison to looting and trading, and war will drain your cash faster than you can earn it. It's only when you're able to take cities that you're really able to reap in the tax money.
Then you'll need a method to build capital, I usually get mine from trading and/or looting bandits then starting businesses in friendly cities. In either method, don't bother spending points in trade or looting, they're better off in leadership since it'll save you money in the long run. If you don't disable companion complaints, you'll have a few "expendable" companions to fill those roles.
Whether to invest in enterprises or not is up to you, but if you plan it right, you can earn your upkeep, starting capital, and much, much more by doing nothing after your businesses start up. If you don't have a mind for numbers, ask the guildmaster for the expected profit range. A profit of around 300+ will pretty much guarantee you won't lose any money there no matter what happens. Of course, a velvet factory in any place but Jelkala is an extra 500+ denars every week if you can afford the 10k starting price tag.
After you've got your starting capital, you'll need to deal with army upkeep. If you were smart and built up leadership, you've dealt with half of the problem. The other half is dealing with recruitment and upgrade costs since you'll have to replace your losses. A companion with high surgery will help
a lot. This, of course, also depends on what setting your "troop damage" option is at.
Remember, no matter what you do, you'll never be able to match other kingdoms in terms of armies alone. You'll always lose a war of attrition, since they can replenish their troops much faster than you can; especially on harder difficulties. Persuade enemy lords to join you whenever possible, since that can work to your advantage.
Only fight land battles when necessary or you're sure you can win with few losses. Only travel with as many troops as you need. If you're not going to attack/defend a castle/town or chase off a lord, you can garrison most of your troops and save the upkeep. A companion with pathfinder is helpful for outrunning larger armies.
Finally, dealing with tax inefficiency. If you have too many fiefs (Towns/Castles/Villages), you'll start losing money to TI. I believe the limit is 6 for "Poor" AI difficulty, 4 for "Normal", and 2 for "Good", with 3% tax loss for every fief above that limit. If you depend on money from businesses, that'll also be lost to tax, but
doesn't count as a fief. So keep the two best money makers and give the rest to your lords.
Once you have a decent number of lords working for you, your troop costs will decrease to the point where you won't have to worry so much about earning money. Start a campaign and use them as shock troops to eat the losses when sieging.
When fighting a war, remember that a long, protracted war will almost always end with you losing. Strike hard, strike Fast, and you'll usually be able to scare an enemy into suing for peace.