Driving WS2811 LED strip from microcontroller
An ATmega pin will happily drive the WS2811 DIN pin, it expects 5V and unless you are driving an obscenely long cable between the ATmega and the first WS2811, you are fine with a direct connection.

The LED strip power depends on how the strip is constructed, but is typically 12V; see the datasheet for examples of both 12V and 5V situations.