Rooftop a/c unit fans runs but when i turn it to ac it trips the breaker. what do i need to do?

If it is tripping the breaker when the compressor is trying to kick on, then you probably have a bad start capacitor. Remove the upper shroud and then the small access panel on the side of the unit. Make sure the disconnect power before accessing this panel. In this area, there are two to three capacitors (depending on the model). The start capacitor has a small black relay on top of it, visibly inspect that for char or burn marks, if there are any, then replace that capacitor and relay(PTCR). If there are no burn marks, then you need to have the compressor checked out for shorts or open windings.

Hope this helps,

Jeff