I recommend to take some basic training by colleagues or your IT department if you are not a consultant, even starting with reading documentation in help.sap.com, F1 field help in SAP, documents and blogs in SCN would be a good starting point.
If you have a reorder point of 80 then you only get new requisitions if your current stock falls below this 80. And you get only requisitions then for the difference between your maximum stock level of 100 and your stock at the time of running MRP.