I am new to arduino and need some help. I just baught light bars for my jeep and wiring harnesses with them, the harnesses come with buttons and I wanted to replace the button and have them controlled on my phone. The harnesses come with a relay and a fuse and a button all 12v, How would i go about using my raspberry pi or udoo x86(which has built in adruino) to basically replace the rocker switch so i can then turn things on and off without needing to have actual switches.
To drive a relay you'll need a transistor (a MOSFET is a good choice) and you'll need a diode across the relay coils to protect the transistor from the reverse current pulse when you remove power from the relay coils.
As Majenko says, if you Google "Arduino relay" you'll find lots of articles covering the topic. (And if you use a digital output line from your PI the exact same circuit will work.)
This MOSFET is a good option for switching high current loads with a 5V logic signal:
The diode will need to be a power diode that can handle your voltage/current needs. You'll need to figure out how much current your light bar uses, and then I'd suggest a diode that can handle twice that much current (at 12V) for a margin of safety.