What do I need for a basic RF circuit?
If you want complete DIY, which is probably not you best starting point, then something like this is about as simple as you can get. Receiver at left. Transmitter at right.



That is from this wonderful page of RF ideas Homebrew RF Circuit Design Ideas These tend to be circuits only with no notes so a bit of a leap for a beginner.

with the actual circuit diagram here



A wireless doorbell may do what you want.



"ASK" RF data modules abound - finding them amongst the 1000 at a  time Alibaba adverisers is the trick.

Here is one at $US6 with free shipping Uses nRF24L01 transceiver IC from Nordic.



From Sparkfun  - uses same IC - $20. 


Description: The nRF24L01 module is the latest in RF modules from SparkFun. This module uses the 2.4GHz transceiver from Nordic Semiconductor, the nRF24L01+. This transceiver IC operates in the 2.4GHz band and has many new features! Take all the coolness of the nRF2401A and add some extra pipelines, buffers, and an auto-retransmit feature - very nice!


You can DIY it with the IC, burt here's what's on board (+ a power supply)




There are many low power short range low cost wireless data modules available.

A large number of idea starters here look for small unhoused modules. 

A bit fancy but would do what you want - but also somewhat complex to use and at GBP13.50 a bit expensive Pololu Wixel

Similar



433 MHz ISM band transceivers.  $15US/1.here





Transmitter $3 free shipping.
Receivers will be ~ similar here





panStamp here

Open source


panStamps are small wireless modules programmable from the Arduino IDE. Each module contains an Atmega328p MCU and a Texas Instruments CC1101 RF interface, providing the necessary connectivity and processing power to create autonomous low-power wireless motes. Everything contained into a compact DIP-24 module.


Store $18.55 basic unit.





Units like this rare widely available at modest cost. This site only sells them in volume (probably).