How can i design a complete scada system?

The basic things u need to know for designing scada system is

1. How many tags to u want read from controllers(total tags)

2. communication protocols.

3. Liecense

4. total Displays.

5. Network Topology

6. Media.... Ethernet IP or Mod-bus TCP/IP etc..

open the scada software go to symbolic factory select what do you need to control and then assign it value and interface it

By "Design",do you mean completely, from the "ground up" as engineers and builders would say.

If so, this is not an easy task as stated by someone else.

First SCADA in an acronym, Supervisory Control And Data Acquisition.

How will it control?

Using what hardware and what programming language.

What does it need to connect to?

Using what protocols?

Do you have drivers for each of these protocols?

Does it use graphics?

What packages are you going to use to create such graphics?

Where will and how will the data be stored?

SQL, OBIX, ......?

Stored where? Local servers, cloud servers..... ?

Will the system need alarm handling?

Timed control of systems?

Data logging?

Graphing?

The list goes on and the technical expertise needed is very high to encompass everything.

So perhaps you can answer your own question.

Do you have the knowledge and the tools necessary to design a SCADA system?

Often we buy in a system (try Tridium EMEA  / Niagara AX) and then develop it into what we need rather than design the system from "scratch".

I wish you well in the task.

First you have to be architecture of the system, then count the inputs and outputs, then comes configuration, At last comes the graphics and software. Once all these are set you can modify. 

If you have a complete idea about what and how we have to monitor and control. its easy