Usually, you don't have to worry about whether to use は or が because
most subjects can be deleted. "You can't get something wrong if you left
it out in the first place." So we'll look at the parts of sentences that
you can delete, starting with subjects.
If you turn to a Japanese person and suddenly make a statement:
明日パーティにいく。
あした パーティ に いく
"[I'm] going to the party tomorrow."
then the listener will assume that the subject is you. So don't bother
supplying a subject. To do so is in fact unnatural; Japanese people
don't supply subjects in conversation if they're obvious.
If you turn to a Japanese person and ask them a question:
明日パーティにいくか。
あした パーティ に いく か
"[Are you] going to the party tomorrow?"
The listener will assume the subject is himself or herself. Easy! Most
one-on-one conversations where either you or the listener is the subject
don't need an explicit subject. So there's no chance of screwing
up は and が.
If you want to make a statement or ask a question about some other
person, use は after that person's name or title the first time you
mention them:
社長は、あしたパーティにいくか。
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?"
Here the は introduces a
change of subject. In this example it
signals a change from the default 'you the listener' to the 社長. After
you establish that you're talking about the president you can go back to
dropping subjects again:
その後は、帰るかな。
その あと は、かえる か な
"Is he going home after that, I wonder?"
Don't be too forward making assumptions about other people. This
prevents the listener from thinking the question is back to being about
themself. There's a strong tendency for questions to erase understood
info and you have to signal that things are unchanged. Usually you play
with the verb a little bit to get this across. Notice that the change in
time being talked about was also signalled with a は.
This tendency to delete subjects in Japanese parallels the behavior of
an English native using simple pronouns such as, 'I', 'you', 'he',
'she', and 'they'. When you comment about yourself you use 'I' (in
Japanese, delete). When you ask about the listener you use 'you' (in
Japanese, delete). When you've first established someone and then
continue discussing that person, you use 'he' or 'she' (in Japanese,
delete). See? It's simple. In linguistics these are called anaphors,
verbal markers which refer to previously established topics. In English
(and in most other Indo-European languages) we use simple pronouns as
anaphors. In Japanese explicit anaphors are not used, instead the
anaphoric position in a sentence is simply left empty.