Assuming you are starting as "No one" and not a vassal noble:
1) If you already have a future faction in mind to join, then you can join a lord's army and follow them into battle for building relations and cheap money. Pick up swords in battle as they are worth a bit
2) Leave after you've got some exp and sell equipment for dosh
3) Go round European cities and asking guild masters for quests; try to get the escort caravan missions; recruit a company of mercenaries and do the missions for easy money and exp
4) Repeat 3) and kill bandits while you can; once you've accumulated money you can invest in enterprises (ironworks, weavery etc) for steady income
5) Once you're happy try to join a faction as mercenary captain, or if you have the renown already, as a vassal
After that, you're in the long game of trying to build up your manor and your army and it's a different story