Ipod where do I start?

Ok now please dont call me a dumba$$ or give me any sarcasm or be like duhhh But I have no idea how to put songs onto my ipod I have had it a year and still havent a clue how to do it I’ve asked around but still am not so sure about it Ok so here’s how I think it should be done

So I have songs that I have downloaded Now if I download itunes I just have to put the songs I want on itunes? Hook up my ipod to my computer Then the songs will go onto my ipod right?

But then, I also heard ipods sync to a computer So like there are already songs on the ipod that my brother put on there before he gave it to me But the songs are sh!t So now, if I connect my ipod to my computer will those songs go and be replaced with my songs?

If im wrong about all this can someone please explain to me step by step how to do it? Ooh yeah and also, is itunes absolutely neccesary? Thanks x