Walkom Alsalam,
I'll assume that you have the proper OPC server that can "talk" to your ABB PLC, Proper OPC server means that it can handle the communication protocol used by your ABB PLC.
OPC server settings:
Step 1:
You should create new "Channel", In this one you configure the communication interface that you'll use between the OPC server and the PLC.
1.1 If you are using a serial connection (RS232-485-422), You configure here all related configurations such as Comport, Baud rate, Parity, stop bits... etc.
1.2 If it was Ethernet, You select the PCI card, IP address... etc.
Step 2:
You should create a new "Device", In this one you configure the communication protocol used by your PLC with exact model if there are many models support the same communication protocol.
Step 3:
You should now create "Groups", I'll talk about it later.
Step 4:
You should create "Tags" or "Items" or "Alias" different names for the same meaning, For each item you create must define the following:
4.1 "Name" for it (This name will be used later in your SCADA software)
4.2 "Type" Analog, Digital... etc
4.3 "Address" which the memory address in your PLC that this tag will hold the value for.
4.4 Other things may be defined depending on the vendor and your needs.
Now what are groups that we defined in step 3?
Answer: Think of it as "Folders" that manage your huge number of tags.
Note that some of the names I used may differ from one vendor to the other, but you should be able to link this info given here with info that you should get from the manual.
Nice regards,
And hope you complete you project successfully.