In this case, you would use the incompleted form "naghahanap". "Naghahanap ako ng challenge kaya nagsimula akong mag-aral ng Tagalog." This is where Tagalog differs from English and other Indo-European languages. English has tenses, i.e. it is important to know whether the action happened in the past, in the present, or in the future. On the other hand, Tagalog has aspects. The question it asks is whether the action had been completed, is incompleted, or is contemplated regardless of when it happened. In your sentence "I was looking for a challenge" the action is not yet finished, hence the incompleted aspect. Hope this clears it up.