I tend to start out doing mercenary work for the Teutonic Order - they'll go at it with the various Slavic factions, who seem prone to war with each other as well. It's good work, you have easy access to high quality gear from the Western European cultures and tend to fight the more lightly armored Slavic troops. It's good money and generally traveling in the company of some pretty rough and tumble lord armies, giving you good odds, decent renown and valuable loot. Iberian Peninsula is another decent choice for similar reasons. When you really establish yourself, get a nice nest egg of money and gear then get involved in the England/France/HRE region. These are some huge military regions; you'll see sieges involving 10-15 noble armies on each side and if you pick your battles right can net you TONS of renown, honor and loot. Also a lot of valuable properties change hands and you can end up with some good cities and castles that way.
Where this takes you:
Build up a good 100-strong heavy cavalry force and keep a separate heavy infantry/archer/crossbow army in your castle/city to take out for sieges. Ride around with your army-eating cavalry and rescue allied nobles getting humped by a good sized enemy horde. Move your cavalry off to the side; ride up and lance down some poor infantry bastard and watch their army charge the middle of the forces of the guy you're rescuing. Send your knights around flank and obliterate their archers, then swoop in and surprise-buttsecks the enemy infantry and cavalry that are engaged with the army of the guy you're rescuing. It'll turn into a route and your cavalry will rack up obscene kills.
Result? Everyone of your faction will love you, release everyone you capture and the enemy will respect you and your honor will skyrocket. You'll be demolishing much larger armies which means good loot, which means good money. You'll end up getting awarded more and more domains.
Do NOT rescue allies cities getting sieged! Let them get taken, then when the attackers ride off crush them in the field where your cavalry will reign supreme. Then hop back, get your archers and infantry, then retake the city from its skeleton garrison. Mmmm.... that renown tastes delicious! When you hit around 1000 renown, marry some daughter of a prince or princess and consider retiring as you've pretty much won.