In general, you don't need it. In the instances where you do, you can slowly build a feel for it. Basically, you need it in situations where you're not expressing a change in subject, but where you want to state the subject even though it has already been established. Usually, this is to add emphasis or to avoid ambiguity.

(Throw in a "みたい" because you don't wan't to seem too certain of the actions of others in Japanese. We're not really covering that though, it's just a side note.) There's no real ambiguity in this case, and "わからない" alone would have worked, but it is a case where you're not changing an understood subject to another, you're restating the understood subject as such for some emphasis. If you're stating an established subject, for whatever reason, use が. But you could have deleted, and if you were following the earlier explanation you should have. (This one sentence also helps to dispell the myth that は is for negative sentences.)