your question is a bit vague. You can add cells from different sheets in a few different ways, depending on how your data is structured.
For example, you can start a total by entering a = sign, then click the sheet and the cell with the first value that you want to include in the total, then type a + sign and click the sheet and the cell with the next value that you want to include in the total, etc., then hit Enter. The formula will look something like
The sheet name is followed by an exclamation mark and then by the cell address. If a sheet name has spaces, it needs to be wrapped in single quotes.
Another scenario is where the values you want to sum up are in the same cell on all sheets. In that case, you can use a Sum function that spans all sheets, like