I have a 30 amp breaker with 10awg wire. I want to connect the 10awg wire to 12awg wire to run 20 amp outlets. Is that OK or do I have to change the 30 amp breaker to a 20 amp breaker?

You will need to use a 20 amp breaker because your #12 is rated at 20 amps or risk a fire or burnt/overheated wiring.

OK thanks. I have another question. I have two 30amp breakers on one 10-2 cable, one is on the black wire and other is on the white wire and there's the ground wire. I want to connect it to two 12-2 cables. I know black goes to black to power one 12-2 cable and black goes to white power the other 12-2 cable. But what about the neutral on the 12-2 cables? The 10-2 doesn't have it since its used to bring power from the second breaker. How do I connect the neutrals?

The 2 # 10's may be a 240 volt circuit.
You will need a second run, you can't use 120 volts and 240 volts from the same cable.

So even when I switch the breakers to 20 amps, it'll still be 240 volts? Doing a second run will be hard. Im trying to use the existing cable. But I don't need 240 volts, which like you said, I think it is. Just two 120 volts for my two 12-2 cables. Is there any safe way to do the neutral wires without having to run new wires??

Voltage is determined by the way the connection is made in the circuit panel.

How many outlets do you need to install?

OK I think I figured it out. I didn't need two power sources, just one so I took out the two 30amp breakers and replaced it with one 20amp. Thanks for your help!