Once you have the active sheet, as you did store in you variable
objsheet
it's really simple:
objsheet.Cells[1,1] = "5";
Getting the value is just the reverse:
double MyValue = objsheet.Cells[1,1];
You could even insert formulas into the cell:
objsheet.Cells[1, 5] = "=SUM(D2:D" + (4).ToString() + ")";
But be sure to type it in English to make it work. It may sound strange, but some Excel versions have a programming language that is in a different language.