I was reading about aspects and I think I understand it's meaning for past tenses, but I still cannot figure out the difference between perfect and imperfect present.
First, only imperfective verbs have the present tense. Your example is a compound predicate: a verb that has different forms of grammatical tense (хочу is an imperfective verb) + an infinitive that has no tense category.
Second, a compound predicate can include infinitives of both perfective and imperfective verbs. The difference between them is equal to the difference between the grammatical aspects: the imperfective verb is the process, the perfective verb is the result.
Я хочу приготовить суп на обед = я хочу результат (суп).
Я хочу готовить суп на обед = я хочу процесс (приготовление супа).
In general, roughly speaking, the imperfective aspect is about state, and the perfective aspect is about state transition.
In this particular case, я хочу приготовить суп is about the result (which in this case is a state transition from not having soup for lunch to having soup for lunch). It means that you want the soup cooked, and you'll be the one making it happen this one time.
Я хочу готовить суп is about the process (which is the state of being at the kitchen cooking the soup). It's more like "I want to be cooking soup" or "I want be the one who is cooking soup".
This phrase doesn't make a lot of sense without context, but it could be used in a phrase like: Я хочу быть домохозяйкой. Я хочу готовить суп, ходить за покупками и забирать детей из школы "I want to be a housewife. I want to be cooking soup for lunch, going grocery shopping and picking kids from school".
It could mean that you want to do this every day, or just imagining the joy of being a person who gets to do all this, but it's the process you're talking about and not the end result.