Adding 0s in front of text to certain length

Select the column, right click on it somewhere, select the "Format cells" option, go to the "Number" tab, select "Custom" format from the list and overwrite the bit that says "General" with 8 zeros.

That will display all of your numbers with leading zeros.

If you actually want to add the zeros to the front and make them text again then insert a column next to the one you've got and, on your first data row put the formula =TEXT(A2,"00000000") and copy all the way down the column (I'm assuming that your data is in column A, starting at row 2.

Once you've copied down then select and copy that entire column of formula, then select the top cell in your existing data and Paste special and pick "Values", this will overwrite your original data with the results of the formula and you can now delete the formula column you added.