How do you make the siren in your car come on, and how do you use your walkie talkies?

That is a very honorable career :) ... Sirens are activated by a switch. Regarding walkie talkies, you constantly listen to them and press a button to broadcast whatever you want to say.

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There are multiple switches for the lights. You can select which ones you want to use.
For example, on a traffic stop, you turn on the overheads, the rear flashers, and the "take downs."
Take downs are two floodlights in the front on the light bar, which, when the driver tries to look back at the police, will temporarily blind him. That is used for officer safety.
If you are driving through a neighborhood looking for an address for a call you were dispatched to, you would turn on the lights at the end of the light bar, called "alley lights."
You also might use the movable spotlight, positioned near the drivers door, near the outside mirror.

Sirens usually have fewer switches. It is more like a dial, like a shower massage dial lets you choose between different settings. There is a regular siren that is used most times. But, if you want to really get peoples' attention, you switch to a different sound. This is used when the police car is approaching an intersection. There is also a button so you can just "bump" the siren when you need it, rather than keeping it on all the time. The siren is very noisy, and makes it difficult to talk on the radio!

Radios have a PTT (push to talk) button, like a regular walkie talkie. However, they also have an emergency button. If an officer pushes down on the emergency button and lets go, that automatically turns on the officer's microphone, so he can concentrate on handling the emergency and dispatch can hear everything that is being said.

Law enforcement is a fun career, and I am glad I did it.
Best of luck to you!