We can't use very 1st and last IP address of a network (like 0 or 255).Last IP address is a broadcast address but why can't we use 1st IP address?

Because1st IP address is the network a address (Network ID)

But first ip address of a subnet may not be "0" also last ip address may not be "255" that depends on the network mask.

I think you are asking whether .0 or .255 can ever be a host address. We can use .0 and .255 as a host address but it depends on the MASK. For example, if you have a subnet mask of255.255.255.0, the entire fourth octet should be for host with the two exceptions of .0 which is the network (this is not assigned to a host) and .255 being the last addess / broadcast address. But if we have a network address of172.16.0.0 /16, the first usable host would be172.168.0.1 and the last / broadcast address would be172.168.255.254. So0 and255 in a octet can be a valid host address. Correct me if I am wrong.

The first IP represent the Network IP where this IP represent all the hosts under this network IP.

The Fist IP is the Network ID.

Despite that you can enable Subnet Zero, But it is discouraged.

You can refer to this link:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

intial ip wont used because it is used for the routing purpose . and its a store for the default routing .

255.255.255.255  is multicast address for the routing protocol rip . so that we can't  assign for our private ip, or even public .

every Ip address has two parts.One for network part and anothet part for host part.In IP address first Ip required for Network bit.Thats why remaining part alotted for Host bit.

network ID

First IP .0 represent the Network ID

.255 would be a broadcasting address

A message sent to a broadcast address is typically received by all network-attached hosts, rather than by a specific host

The first @ is the @ of the network Id , and you can not use it , it is reserved ..you can use it in routers by activating subnet zero ..

First Ip address is the Network Address (network id), it identifies the network subnet mask , e.g if the network mask  belongs to255..0.0.0 that means its class A network , so the address's must be belongs to class A .. example :10.0.0.0

1st IP address is the network address (Network ID) of a subnet.

In case of sub-netting,first ip address of a subnet may not be "0" and last ip address may not be "255".