さ / さあ

At the start of a sentence, さ or さあ gets things moving: "right", "come on", "now then". You say it just before you do something, or to push someone else to do it. It is casual.

さあ is the stronger of the two, and doubled it hurries someone along.

さ in the middle of a sentence

In casual speech, さ also goes after a phrase in the middle of a sentence – a topic, a time word, あの – as a filler, like "you know" or "like". It holds the listener's attention while you get to the point.

This さ belongs with friends and family. In polite speech, or to a superior, leave it out; あのさ becomes あの or すみません.

さあ, "who knows"

Said slowly in answer to a question, さあ means you don't know or can't say: "hmm, no idea".

At the end of a sentence, さ is a different use, a sentence-ending particle.