Subjects and Deletions
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:
あした パーティ に いく
"[I'm] going to the party tomorrow."
If you turn to a Japanese person and ask them a question:
あした パーティ に いく か
"[Are you] going to the party tomorrow?"
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:
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?"
その あと は、かえる か な
"Is he going home after that, I wonder?"
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.