There are a bunch of basics that you need to get under your belt
- Dealing with digitals - reading/writing, pullup/pulldown resistors, using switches
- Dealing with analogs - reading/writing, PWM, voltage dividers (dealing with a whole class of sensors)
- Using the Serial output for debugging
- Learning how to do lots of things without pausing (no delay()) - see blink without delay and learn that technique
- Learn about Finite State Machine style programming
Then it depends on what your interests are - find a project and put it all together into something useful.