Not sure if you really mean brackets or you actually mean parentheses
parentheses ( ) ; brackets [ ] ; braces { }

If you really mean parentheses, here's a simple way. First highlight
the cell(s) you want formatted this way, then:
1.)Do the menu sequence FormatCells
2.)Choose the Number tab
3.)In the Category field, choose Number
4.)In the Negative Numbers field, pick either of the 2 numbers (black
or red) enclosed in parentheses. (Optionally choosing Decimal Places
and 1000 Separator while you're in that screen.)
5.)Now, any cell containing a negative number within the range you
highlighted will be enclosed in parentheses.

However, if you really do mean brackets and not parentheses, it's just
a little more complicated. Again, highlight the cell(s) you want
formatted this way, then.
1.)Do the menu sequence FormatCells
2.)Choose the Number tab
3.)In the Category field, choose Custom
4.)In the Type field, enter #,##0;"["#,##0"]" for no decimal places or
#,##0.00;"["#,##0.00"]" for 2 decimal places. (Be careful to include
those quotation marks as Excel doesn't allow entering brackets in a
custom format without them!)
5.)Now, any cell containing a negative number within the range you
highlighted will be enclosed in brackets.