on the cell, right click and go to Format Cell
There will be a tab labeled Number (the first one)
select the appropriate Category from the list (Time, in yor case)
choose one of the formats listed. (one of the bottom two, at least in Office 2003)
If none of the formats matches what you want, select one that is close to what you want, then choose the Custom category. You can now edit the format however you choose using the format you chose to customize and the other pre-defined custom formats as a guide