Editing the config is simply a matter of changing a few things in the file by opening it with Notepad and then resaving it.

To find out what all the stuff is inside the config, open one of the default aircraft config files and take a good look at it.  They're labled and given a brief explanation.  If you need more help, check out the FS2004 SDKs.  The link is in my signature.

Janaki,
Is this what you are looking for:
//0  Class   <0=none,1=wheel, 2=scrape, 3=skid, 4=floats, 5=water rudder
       //1  Longitudinal Position        (feet)
       //2  Lateral Position             (feet)
       //3  Vertical Position            (feet)
       //4  Impact Damage Threshold      (Ft Per Minute)
       //5  Brake Map     (0=None, 1=Left, 2=Right)
       //6  Wheel Radius                 (feet)
       //7  Steer Angle                  (degrees)
       //8  Static Compression           (feet)  (0 if rigid)
       //9  Max/Static Compression Ratio
       //10 Damping Ratio (0=Undamped, 1=Damped)
       //11 Extension Time               (seconds)
       //12 Retraction Time              (seconds)
       //13 Sound Type
       //14 Airspeed limit for retraction     (KIAS)
       //15 Airspeed that gear gets damage at (KIAS)
There is quite a lot on things that get adjusted here, but some are 'set' and should not be changed. Also, leave out one period, or one comma, and your AC will go crazy (spins, tire marks, etc).

After you become familiar with what does what, you can put 'floats' on a Bell 206, or make if 'act' like a 747.
Chuck