Arduino 'Examples' within the Arduino IDE, is the first Step.
Working with sensors, is yet another simpler task that can be accomplished by following the connection diagrams(Fritzing) along with their codes, easily available allover the Internet and Community.
Download the Arduino IDE.. The programs that run the Arduino are called sketches these sketches can be found in the Arduino IDE software that you downloaded.. Look for samples. You will see basic code such as "blink" blink is a code for blinking a basic LED at different speeds etc.. when you click on this sample code a window will pop up showing you the code that will be loaded to the Arduino board / module. Read the text ! there are literally dozens of videos on youtube that show you how you can edit the code to do all kinds of things like making the LED blink faster etc. Watch lots of videos based on Arduino.. you will be really surprised at the things you can accomplish.. You will hear about Processing just stick with the basics for a few months. Processing is the next step.. You should have some understanding of Circuits, if not You should learn about building circuits before getting into Arduino.
The architecture of Arduino UNO (Read about ATmega328P also)
The architecture of Arduino programming ( understand the stepup and loop functions)
Read upon GPIO functions and Pins ( General purpose Input Ouput)
Now you compile and run the blink sample code
Now take one more step into peripherals - UART (serial communication); What happens when you use Serial.print() function
Learn how to change the blink intensity by varying voltage (PWM - Pulse width modulation)
How to communicate with external sensors ? Using SPI, I2C or UART
How to control external transducers
The best way to learn is by doing some project. Take a real world problem and try to make an Arduino based solution for it. This may require multiple sensors and transducers. That will make you learn about different communication protocols. Then you will learn how to use them with the Arduino.
