Hai..
I hope u are biginer for siemens s7 , I too but what ever I have idea I would like to share with u.
I hope u have proper programer with s7 software and communication cable to communicate with plc(RS232 to MPI or USB to MPI adopter).
Connect to the plc on which u want to go online, open new project and on above tool bar go to PLC and click upload ( Don't download in any care), if ur comm port is working properly ur plc station will upload in ur PG, open s7 program and then blocks u want, after open blocks on tool bar there will be a google icon to monitor click on that u can moniter ur PLC on line now(you can see a green bar will flash on bottom tool bar). You have to do same for each block u want to moniter.