Acting as a 接続詞（せつぞくし） (conjunction),
さて is a word used when one wants to change the topic of conversation, or is ready to move on to a new point. Usually it is translated as ‘well’, or ‘well then’.
さて will appear at the beginning of a sentence, before the speaker highlights what the new topic is.
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さて、そろそろ出（で）ますか。
Well, let's head off soon.
さて、とりあえず乾杯（かんぱい）しましょう。
Well, first things first, let's have a toast.
Although
さて is extremely rarely written in kanji,
扨（さて） or
偖（さて） may be seen every now and then. Both of these kanji have the same meaning of ‘now’ (now being the beginning of something new).
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扨（さて）、この問題（もんだい）の答（こた）えが分（わ）かる人（ひと）はいますか。
Now, is there anyone here that knows the answer to this question.
偖（さて）、その荷物（にもつ）をこの箱（はこ）の上（うえ）においてください。
Now, please place that luggage on top of this box.