我想吃鱼，means I want to have some fish.It emphasizes the desire of yours. 我要吃鱼，means I will have some fish.It emphasizes the action which will happen in the 我喜欢吃鱼，means I like fish.It emphasizes the preference. Of course ,when you have a dinner in a restaurant ,you can use any of them to show what do you want to have. But when you talk with somebody about your favorite food you need to use 我喜欢吃鱼。

要=want to do sth. 想=think of doing sth 喜欢=like to do/ doing

Hello Romulus, When you use " 要yao " it means you are asking for something you want,your expressing your need for it to be fulfilled by demanding it. It is appropriate to use in a restaurant asking the waiter for it. When you use " 想 xiang " you express the desire to do or have something. It is an instant desire that is supposed to be fulfilled by expressing it ,so it is likely to be used in a restaurant as well asking for the fish by expressing the desire to eat it. "喜欢xi huan" expresses liking something. It is a general liking or favoring of something or someone.

yao= want to/would like to xiang= would like to/ also: to miss (somebody) xihuan= like you use yao, when the next word is a subject and xiang when it´s a verb. Here are some examples: Wo yao rou. = I want/would like some meat. Wo xiang chifan rou= I would like to eat some meat. Wo xiang wo de baba. = I miss my father. Wo xihuan ti ziqiu.= I like playing soccer. I hope I helped you with that.

想（xiang),mean ,you sit one chair and use your head .your cerebrum is work ,but your hands and your mouth is close. and you do not speak ,and you do not to do it .