How do all these girls get their hair like this?

That actually looks natural to me. If you have naturally wavy hair then all you have to do is scrunch in a soft hold curl enhancing gel while it is wet and then don't touch it until it is 100% dry, then if it is hard, scrunch it lightly to get the crunch out, and you should have soft waves. If they are too compact, or chunky, then lightly finger- comb through your hair and it should look pretty close to that. I do this, but I have naturally wavy hair, so it doesn't take much work for me. If your hair is naturally straight, then you've going to need a curling iron and a TON of hairspray, and you'll probably have to comb it out to get it to look like that, which will make the wave fall out because straight hair is nearly impossible to convince to hold a curl. Or you could try braiding it in long braids while it is wet before bed and take the braids out in the morning, spray a lot of hairspray on it, and then try to break up the waves a bit, if it doesn't take the curl out, it should like almost like that:)