Lots of people can probably help you but your question is so vague, in its simplest terms the answer is by writing them !!
A few questions to consider for instance, how many bits of data, how historical does it need to be, how do you want the report presenting, PDF, Excel, text file etc. How is the report run, from a button in Citect, from a web browser etc. etc.
You can generate reports fairly easily using the standard Citect reports and events, have you read the manual ? If you need rich reports with lots of data and charts then you may need to look at the dedicated reports package.
Have a look at the standard functionality and come back with questions after that and i may be able to help a little
Cheers