Generally, what you start as is dependent on three factors.
1. What the faction needs to most.
2. Where your competency within writing and fleeting lay.
3. What you want your character to actually do.
For instance. When i created Boan we had several fleeters, i was a very experienced writer in fleeting, and i wanted Boan to already be in charge of something. As such i made her a Commander, in charge of a single ship. This is generally viewed as okay because in the grand scheme of things it is not a big deal.
As for your character, i would start off as a lieutenant or perhaps a second in command, giving yourself time to acclimate to fleeting and such.