How to make a formula used in multiple cells, take a value for one specific cell only

Sometimes, for simple simulation problems excel can be quite convenient. However, what I do not know is how to make a formula used in multiple cells to take a value for one specific cell only. What I mean is this:
I have my parameters as well as my variables. I then create the formulas and I copy/paste in order for example to generate an artificial series. However the formula as it is copied and pasted, it is updated by the new values that the variables take as we move down the spreadsheet. But sometimes the formula will contain parameters i.e. values that are to be taken from one specific cell only. Hence my question is how to "lock" this reference cell in the formula so that when I copy and paste it it remains the same i.e. cell B4.
Can anybody help?