All characters must have as their object name (the name you see on screen when you talk on channels or pose), their formal or family name. For most characters, this will be their last name. For Bajorans and some characters of Asian descent, it would be the "first" name. This means that your name should be whatever would come after "Lieutenant" if someone was addressing you by rank. So, for instance, James Kirk is Kirk rather than James. Kira Nerys is Kira rather than Nerys.
Another option is to have your full name as your character name (i.e. "James Kirk")... as long as your formal name is somewhere in there.
This policy extends to all characters. Starfleet, UFP Civilians, Aliens, NPCs... it matters not.