I2C is a particular approach for connecting together chains of devices. It's different from the typical way we have connected things to our Arduino. Here's what you need to know regarding connecting I2C devices.
An I2C device needs four wires to work. Two of them are our usual VCC (5V) and ground. The other two are labeled SDA and SCL. They're located where pins D20 and D21 should be. The labeling can be hard to see, so look carefully.
You can't connect two I2C devices of the same type to the same Arduino (i.e. two accelerometers), but you can connect multiple I2C devices of different types (i.e. an accelerometer and an LCD screen). If you do this, just connect all of the SDA wires together, and connect all of the SCL wires together.
I2C devices are very very sensitive to random electrical surges. If such surges occur, they'll freeze your program and you'll need to restart it by clicking the round reset button on the Arduino. Here are two key strategies that are important in reducing the likelihood of these surges. You MUST do these if you plan to use I2C devices.
1. Keep all of the I2C wires, including VCC and ground, away from each other. The worst thing you can do is use a ribbon cable of 4 wires all running alongside each other. Build your device so that your four wires are as far apart from each other and other wires as possible. If you don't believe that, watch this video where I demonstrate that just holding some I2C wires together makes a wiggling servo freeze up.
2. Power your servos externally, rather than through the Arduino. Each servo has three wires: signal, VCC, and ground. An Arduino pin should be connected to the signal, as usual. But VCC and ground should be connected to an external 5V power supply. You then also need to make sure to tie the two grounds, i.e. connect the ground on the Arduino with the ground on the external power supply.