There are many options to measure temperature for your projects. One of the easiest method is the use of the TMP36 Chip. It is highly accurate with a temperature range between -40°F to 302°F (-40°C to 150°C), easy to come by, and is very cheap.
Step 1: Parts
Step 2: Connections
The TMP36 has three pins. Looking at the flat side of the sensor the leftmost pin is Voltage in (between 2.7V to 5.5V DC). The center pin is Voltage out. This puts out a variable voltage between 0.1V (-40°C) to 2.0V (150°C), and will connect to any of the Analog input pins on the Arduino. The right most pin is Ground.
In this example I have connected the follwoing TMP36 pins to the Arduino respectively, Vin to 5V, Vout to A0, and Gnd to Gnd.
Step 3: The Sketch
Use the attached sketch (TMP36.ino) and upload it to your arduino. After loading the sketch use the serial monitor (Tools > Serial Monitor) to see how the TMP36 responds to its environment.