First, you need the help of experts - go collect your companions. OK, they are not experts yet, but you need to assign each a role and build their skills. Although they may grow to be good warriors, it is their party skills that you really need. Get a medic an engineer and a scout. Someone with good strategy is useful. The sooner you get them, the sooner they will level up. Don't forget to give them trainer skill once they get to 10-ish level - it will make building armies and replacing the dead MUCH easier.
Second, you need cash. Tournaments are a good start, just pick the cities that have tournaments that fit your style. I usually have low or no power draw and no mounted archery, so tournaments at the Khergit cities tend to be more challenging. The Nord and Rhodok cities offer more foot melee and Swadia offer more mounted combat, Vaegir have more foot archery in the mix. Ask you local arena master about where to find tournaments.
The other way to make both cash and some XP is to fight bandits. Sea raiders are lucrative in term of loot, but are difficult and will cause many casualties on your side. Mountain bandits are easy if you know how to counter their mounted units (light horses, pretty easy). Forest bandit are a pain if you don't have many shield troops. Against shields or armored cavalry they are meat. Use some blunt weapons to get prisoners - sell them, or talk to them in the party screen to get them to join you and fill your ranks. Do not piss off any nation until you are ready to begin your conquests. Then you can raid the caravans of the nation you want to attack for some cash and to start a personal war with them (so you can siege them).
When you have some cash (don't spend too much on gear, there will be time to get the l33t stuff later. A decent armor and some weapons are enough) start recruiting mercs like mad. Get the one in the taverns and ask the bartender for lowest tier troops. If you (and companions) have some trainer skills, they will become 3-4 tier mercs very fast and it is cheaper than recruiting professionals. Fill your party, then form a regiment (talk to a troop in party screen, then on the world map talk to this new unit to transfer more troops). Fill the regiment, tell it to follow you and get even more troops. Just remember that you want them for siege war - mounted troops have no advantage in that respect and foot mercs are cheaper. Then start your conquests.
First thing you build in your new home is barracks or a range to recruit home troops. If you have this in the castle/town it will start recruiting garrison even when you are not there, supplying you with an endless stream of fresh blood and prevent the castle from being sieged by puny lords. From this point on, the world is your oyster.
