So that it uses the same value in a formula …
Your question is not clear, guessing you mean how to stop a cell reference in a formula from changing when you copy that cell to a different cell in the spreadsheet. In this case use a $ sign prior to the part of the cell reference that you do not want to change.
If the formula in cell C1 of Sheet1 is
= A1
And you copy cell C1 to cell D2 of Sheet1 the formula in D2 will change to
= B2
If the formula in Cell C1 of Sheet1 was
= $A$1
And you copy cell C1 to cell D2 of Sheet1 the formula in D2 will still be
= $A$1
You can use the F4 key to add the $ signs after selecting the cell reference when you enter your formula.