http://mountandblade.wikia.com/wiki/Marshall
It should work like in Native Warband, you have to get elected, and I have seen diffrent generals discuss who they want to be marshal.
Also for the uniform, the highest I found was a Lieutenant uniform, at the rank of Colonel.

haaa, thank you my friend, because I was see a player he play with a marshall uniform, me I have the Lieutnant uniforme but I try speak with lot mareshall but they say " not need of me for the time " ... mmh is shearch

To get a marshal's uniform, you need to attain the military rank of marshal in your faction, which is tied ot your renown.

sorry for my English ^^ yes I understand but when I go the Depot for choice my uniforme unity the lats grad is Lieutnant, ( i'am Sorry because I begin only on mount and blade mod Aigle ) do you have a screen for me ?

引用自 Guli Show：

sorry for my English ^^ yes I understand but when I go the Depot for choice my uniforme unity the lats grad is Lieutnant, ( i'am Sorry because I begin only on mount and blade mod Aigle ) do you have a screen for me ?

do you speak french if so I can help you

oui, peut tu m'expliquer ??merciii

enfaite je t'explique, j'ai le mod Aigle, et j'aimerais avoir l'uniforme du marechal, mais je ne sais pas comment faire, car lors que je me rend au dépot le dernier grade que je puisse trouver c'est Lieutnant, Sergent, officier, Ranker.
et vue que je débute seulment sur Mount and blade, j'ai du mal a comprendre le jeux.

ah ok le truc c'est que tu dois avoir plus de "renown" car tu ne peux pas acheter le titre de marshall il faut le gagner en sortant victorieux des battaile une fois que tu atteint un certain niveau tu pouras demender une promotion au dépot.

ha merci maintenant c'est plus claire :))) et il faut combien de renomé ? pcq j'avais mis les codes pour avoir le level 62

Work very very hard! I made Marshal very fast from being very agressive! Actually when i left France i was INVITED to be Marshal of the British Empire by the King, but obviously that takes a lot of work!