Generally, I start with basic training, which makes me level 2, then hit the tourney until sometime after level 3. Now that I am more practiced, I can make money in the tournament as a squire base. You can start as a trader base instead, which gives more starting cash and goods, and better charisma and trade skill, but makes fighting tougher.
Once you work up a basic stake, you start working the merchants. Don't hire any men, and don't buy more than one or two items at first. This will keep your overland speed above 9, which will outrun everyone. Don't take any quests, just run goods, and add cheap (lame or sumpter) horses to your inventory, as this will increase your carry ability without reducing speed. Join battles in progress for XP; note that you can always leave these battles. Retreat if things are going badly, don't be KO'd as the last man standing, and you will lose nothing.
You should be able to solo the River Pirate groups, which is another source of XP and weapon skill powerups. Take the quest, it's only 125 extra gold total, but you'll be killing the pirates anyway.
Once you have a couple thousand in gold worked up, get Marnid (he has trade 3 to start) and Borcha (he has tracking and spotting to start, which makes dodging unwanted fights easier). Make sure both are mounted. You will level some off of fighting XP; use some skill points to increase your inventory skill to hold more horses and goods. Be careful when traveling to the west or the north; Black Khergit Raiders and Dark Knights are relatively fast overland, so keep your cargos small and your eyes open there.
Once you have enough cash for a good horse (spirited saddle horse or better) and a lance, hire a few men (this assumes you have mastered the couched weapon, if not, practice on River Pirates) and escort a few caravans. Don't get fancy, just get them there. This will open the other merchant quests, and you can dump your slow footmen or recruits and take delivery and kidnapped girl quests that allow you to keep your speed up, while making both money and xp.
After a bit of this, you should have leveled up yourself, Marnid, and Borcha quite a bit, and you should have gotten decent equipment and mounts. You also should have a decent string of packhorses, some cash, and perhaps even a few rescued horsemen and knights. Now you can start hiring footmen or militia. Once you have a full company (remember to leave at least two troops slots open, and be one at least one man down, so you can promote troops and still take the kidnapped girl quests), level them up with troublesome bandits.
At this point, you should have a powerful military force, good equipment, excellent allies, and a large cargo capacity. Now you can do pretty much what you want.