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.
しゃちょう は、あした パーティ に いく か
"Is the president going to the party tomorrow?"
"No, it doesn't look like it."
(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.)
"I really don't know why."
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.)