If you are a vassal of a faction you gain rtr every time that faction makes peace with another faction. So stick around for a while
Also mercenary is you being hired by a faction for a certain amount of time. The only gain you get from this is improved relations with lords (which you'll need when you start your own faction), some money and experience. You will not be rewarded with land and you will not gain right to rule.
Actually, you do
gain RtR as a merc whenever the faction you're working for makes peace. As a merc, the faction's enemies become your enemies, so you can get in some combat practice, as well. Mostly, though, the marshal has you running a bunch of errands, generally scouting and gathering food (the dreaded cattle quest).
You have to be an independent to become a merc, though, so if you're already a vassal of a faction your best bet is to stay there; a low-level vassal does much the same things that a merc does. It's good to do when you're leveling up, before you join a faction.