one way to create instance is while installing db2 DB2 instance is created
but what is the syntax of creating instance after installation

try searching for stuff in DB2 information center you will get most of your queries answered....
DB2 Information Center Link

btw this is what i got there

Procedure

To create an instance using db2icrt :

1. Log in as user with root authority.
2. Run the db2icrt command. For example:

DB2DIR/instance/db2icrt -a AuthType -u FencedID InstName

DB2DIR
is the DB2 installation directory.
* On AIX, the DB2 installation directory is /usr/opt/db2_08_01
* On all other UNIX-based operating systems, the installation directory is /opt/IBM/db2/V8.1
-a AuthType
Represents the authentication type for the instance. AuthType can be one of SERVER, CLIENT, DCS, SERVER_ENCRYPT, DCS_ENCRYPT. SERVER is the default. This parameter is optional.
-u FencedID
Represents the name of the user under which fenced user defined functions (UDFs) and fenced stored procedures will run. This flag is not required if you are creating an instance on a DB2 client. Specify the name of the fenced user you created.
InstName
Represents the name of instance. The name of the instance must be the same as the name of the instance owning user. Specify the name of the instance owning user you created. The instance will be created in the instance owning user's home directory.

For example, if you are using server authentication, your fenced user is db2fenc1, and your instance owning user is db2inst1, use the following command to create an instance on an AIX system:

/usr/opt/db2_08_01/instance/db2icrt -a server -u db2fenc1 db2inst1