1: Buy a USB programmer. There are other ways, but this is the easiest. They cost like 1$. Example
2: You need to make a change to your USB Programmer if you want to push code to it. You will need to short these pins when you upload your code into it:
I recommend you to solder some kind of switch to it, to be able to switch between "code mode" and "debug mode". The debug mode will allow you to use the Serial Mode. The code mode will allow you to push code to the microcontroller and to do that those pins need to be shorted. My solution: