Playing as a mercenary for a lord is done by talking to your lord of choice and choosing the 'I would like to enlist for your army' option. Though being enlisted can be very rewarding, it can also make your mid game very frustrating if not done properly. Be careful around Khergit and Sarranid cities, since the mounted bandits patrolling these lands move particulary fast on the overland map. Usually it is preferable to do this without an army, since this makes it easier to dodge the bandit parties lingering between the cities.
In case you decided not to use the ' Gather companions' options, the very first step of most new characters will be to go from city to city to gather some base of companions. Although these paths are all unique, they boil down to the same goals: Setting up some form of steady income, companion hunting and getting gear for your character and companions. The Floris mod allows for several ways to start the game with your new character.