Prepositions in front of an infinitive are not related to the infinitive verb itself, but they are needed to specify the type of information conveyed by that non-main (i.e., not the verb nor the subject) part of the phrase.

In your examples:

 - "ho deciso di portare" -> is split as "ho deciso" (I decided) and the verb "decidere" needs a part-of-the-phrase with the preposition "di" to convey the information of what was decided, if it was an action.
 - "vado a prendere" -> verbs of motion typically require the preposition "a" in parts-of-the-phrase that tell where somebody moved (if the location does not contain. In that case, the preposition would be "in"). You can find it also with non-verb destinations: "vado a casa", "vado al mare".
 - "voglio chiedere" -> as is typical in many languages, direct objects do not need a preposition. What do you want? I want to ask (voglio chiedere). In this situation, "chiedere" (to ask) is direct object, so it does not need a preposition.