It is hard to get numeric data into a RaspberryPi through the GPIO because it does not have an analog to digital converter. This instructable shows how to time the charging and discharging of a capacitor to read a resistive sensor. The demonstration uses a photocell and a force sensitive resistor to control the brightness and blink rate of an LED, and it shows how to use millis() so changes happen instantly instead of having to wait for delay().
This is not a true analog to digital converter.
You will need a RaspberryPi, a breadboard, and a way to run wires from the RaspberryPi to the breadboard. You can use male/female jumper wires but one of the Pi Cobblers listed on this page from Adafruit will make it a lot easier: http://www.adafruit.com/search?q=cobbler
Adafruit Assembled Pi T-Cobbler Breakout for Raspberry Pi: Now that you’ve finally got your hands on a Raspberry Pi® , you’re probably itching to make some fun embedded computer projects with it. What you need is an assembled add on prototyping Pi T-Cobbler from Adafruit, which can break out all those tasty power, GPIO, I2C and SPI pins from the 26 pin header onto a solderless breadboard. (read more)
Each Friday is PiDay here at Adafruit! Be sure to check out our posts, tutorials and new Raspberry Pi related products. Adafruit has the largest and best selection of Raspberry Pi accessories and all the code & tutorials to get you up and running in no time!
Make a robot friend with Adafruit’s CRICKIT – A Creative Robotics & Interactive Construction Kit. It’s an add-on to our popular Circuit Playground Express, FEATHER and other platforms to make and program robots with CircuitPython, MakeCode, and Arduino. Start controlling motors, servos, solenoids. You also get signal pins, capacitive touch sensors, a NeoPixel driver and amplified speaker output. It complements & extends your boards so you can still use all the goodies on the microcontroller, now you have a robotics playground as well.