I am going to do something = means its just an action or a possible action I want to do something = a desire to do something. ------------------ I am going to paint my house = that is what you plan on doing in the future. (am going to = future) I want to paint my house = this is something you want to do but might not actually do it. ---------- going to = an action that might/will/could/plan on taking place. want to = a desire to do something but might not actually happen. another example: I am going to be a millionaire = might happen / will happen / or is a plan or goal you have i want to be a millionaire = its a desire but doesn't mean you will take action in becoming a millionaire -------------- I am going to do something does not mean that its something you want to do but have to do i want to do something does not mean its something you will do. example: - I am going to go to work. you might not want to go to work. but its something you have to do for a specific reason - I want to go to work. means you enjoy your job or you prefer going to work rather than doing something else - I want to travel the world. Doesn't mean you actually will, maybe because you dont have the money, or time to do it. - I am going to travel the world. This means you have the money to do it and the time. or you are putting things in place to make this happen, you are saving money, doing research on different countries, maybe learning different languages, etc.

1- I am going to do something is simple future tents 2- I want to do something is an infinitive phrase. you are confusing why both of them have "to do". The phrase No.1 could be write in this way too "I will do something." I hope you know "will" and "be going to" are same meaning. The structure of phrase No.2 is verb "want"+ infinitive "to".

I am going to do something : it means you will do it, for example : i am going to make a cup of tea, i want to do something : it doesn`t mean you will do it, just saying you want it. for example : i want to go on a trip.

what do you mean with sth ?