Which Arduino board do you use ?
Are you connecting 3.3V sensors with a 5V Arduino board ?
How many sensors and modules are connected to the I2C bus ? Which modules are they ? Can you give links to those modules ? Are there pullup resistors on the modules ?
The first thing to try with a I2C bus is the I2C Scanner: Arduino Playground - I2cScanner.
A Nokia 5110 display runs at 3.3V. Sometimes they are sold as being compatible with 5V, but they are not.
Sparkfun hookup guide: Graphic LCD Hookup Guide - SparkFun Learn
The Arduino uses normal C++, and the special Arduino functions are here: Arduino Reference - Arduino Reference. In the menu on the left on that page you can also chose "LIBRARIES" for the standard libraries that are already included with the Arduino IDE.
The difference with a Arduino sketch and embedded systems is how interrupt are used. With the Arduino it is often not needed to use interrupts. There are other ways to achieve the same goal. For example with millis().
@PaulRB, Scratch and Arduino go well together. Kids all over the world are using it. There are a few Scratch-alike applications. For example mBlock. There is also Tinkercad.com, which has a "Circuits" section for Arduino. It can use "Blocks" and normal Arduino text for a sketch. There is no need to install something, only creating an autodesk account. I did not even bother to try my millis-demo on a real Arduino board :o