Arduino and Raspberry Pi are wonderful platforms for learning electronics. There are plenty of sensors and add-ons you can use to enhance your creations. If you are just getting started, you may want to give the CanaKit Raspberry Pi 2 Starter Kit a look. It comes with a Raspberry Pi 2, WiFi adapter, power supply, case, LEDs, switches, and other components to help you get started. You also get a 8 GB MicroSD card. Costs under $100 at this time.