"La voy a preparar."
Translation:I am going to prepare it.
Can i still say voy a preparala?
You can say "Voy a prepararla."
Since when did la come before voy?
If you mean as opposed to before "preparar": The informal future "ir + a + infinitive" structure is treated like a verb phrase, so the direct object pronoun must precede the first verb.
If you mean as opposed to attached to "preparar": When the verb is in the infinitive object pronouns may be attached to it, but this is optional.
I take it you mean because in English we normally say "I'm going to prepare dinner" with no article before "dinner." Spanish is being accurate, English is being lazy. The dinner in question is something specific, so we should say "I'm going to prepare the dinner," but of course we generally don't. In defence of English it could be suggested that when we drop the article we are using "dinner" adverbially, or as part of a verb phrase at least, treating "Prepare dinner" as a description for the entire action.
As an aside, while Spanish may not do the same in this case, they do have specific verbs for eating the main meals where we would normally resort to a verb + noun construction. "Voy a cenar"="I am going to eat dinner" or "Voy a almorzar contigo"="I am going to have lunch with you." Of course, we have the verb "dine" but this could refer to any meal, and we can say "I'll lunch with you" but this isn't standard English ... yet.