this link you'll find what you need
good luck

http://trainworld.us/NF2/English/Fleet_SQ.html

you need a radioman capable of forming a fleet (lvl 51+ iirc)
then you will also need 10+ people willing to join your fleet

you need a 20x20 BMP image of your fleet tag logo symbol


when you form your sqd into a fleet .. you will be asked to upload your tag (fleet mark)

I used to be a fleet leader on the new york server,and here are the full details:
You need:
Level 50 radioman classed as "cno"
1,000,000 credits
To be a squadron leader with at least 10 members in your squadron.

Your emblem MUST be:
20x20 pixels
Saved as FleetMark.bmp (capital F and capital M-the game will NOT recognise it otherwise)
Saved in your navyfield folder.

Once you've done this,you'll need to restart the game.

Above was almost correct, except the capitals are not required..

Bit more info (Most logos have transparency rather than a white or black background, for this the pixels you wish to be transparent must be 000000)

The fleetmark.bmp must be saved in the root Navyfield folder, usually (c:/programfiles/sdenternet/navyfield) or similar.

Once you have reached all the requirements listed in the posts above, right click radioman, click on clan, and you should see "register seal" just below the "fleet bank" click on this, and confirm. You will then need to reconnect to see the logo.

Its O.N.O @ lvl 50 not C.N.O

C.N.O is lvl 80 for declaring HAs.

I should know I've leveled enough of them for 10 fleets with 5 squads each lol