I’m not that familiar with Atmel but the concept is the same among microcontrollers. I taught myself the Microchip PIC microcontroller and what I did was purchase a PIC programmer. I’m sure there is something similar that is for Atmel. I purchased a microcontroller that I wanted to experiment with…………preferably a dip type package so that I could prototype on an experimental proto board. You need to have a power supply to be able to power it up. Download the data sheet for the microcontroller. This should also have the op codes or instructions that can be used. I downloaded the free MPLAB-X assembly software where you generate your code, assemble it and then send it off to the programmer. I’m sure there are similar products and items for the Atmel.

It really helps to have a project that you want to do. That motivates one to actually learn when there is an end result. I also found lots of code examples on the internet. The first thing to do is just to get to the point where you can actually toggle a bit and read a bit on a port.