Which verb is usually used to say: "I usually wear casual clothes."? I've learnt 4: "indossare", "vestire", "portare" e "mettere", and what are the differences among them?
In the specific case of your sentence, I'd probably simply translate «In genere mi vesto casual» (yes, casual is used in Italian too).
As for the four verbs:
I'd add an example to clarify the different uses of mettere and portare:
"Non porto occhiali" (notice the absence of the article "gli" before occhiali) means "I don't wear eyeglasses" as an 'absolute' statement, because you have a great sight or whatever.
"Non porto gli occhiali" can either mean what I said previosly or "I don't bring the eyeglasses", meaning you don't phisically bring them with you.
"Non metto gli occhiali" means "I don't put the eyeglasses on". Generally speaking, when it comes to mettere used it this kind of context, you can always translate it with put on.
As a native of both languages, the best way to describe their usages would be as such:
Indossare - to wear (formal, for writing) Eg. In formal situations, Romans wore togas.
Vestirsi - to wear Eg. I like to wear clothes that are environmentally friendly (broad).
Portare - to wear I wear sunnies even when it's cloudy (informal).
Mettersi - to put on It's so bright today, I'm gonna chuck on my sunnies (informal).