Learn Dutch: Wij zoeken naar de tas. - We are searching for the bag.

You would use the question word wat (what). The question becomes Wat zoeken wij? Notice how in a question, the verb zoeken swaps places with the subject wij and comes right after the question word.

I thought 'for' was translated as voor. Why does this sentence use naar?

It is because of the verb! In Dutch, the verb zoeken (to search or to look) naturally pairs with the preposition naar. This is a fixed combination (zoeken naar), similar to how in English we say 'listen to' instead of 'listen at'.

Could I say we zoeken instead of wij zoeken?

Yes! We is the normal, everyday way to say 'we'. You use wij when you want to add special emphasis to who is doing the action, as if you are saying 'We are searching for the bag (not someone else).'

Is there a trick to knowing that tas uses de instead of het?

Tas is a common-gender word, which means it takes de. There is no simple rule for guessing this for most words, so it is highly recommended to memorize nouns with their matching article right from the start: learn it as de tas.