"ki" or "sa" as "that" as the subject of a sentence?

I think you mean the relative pronoun.  Am I right?

The lady that was here.

Madanm ki te la a.

That's the car that hit me.

Sa se machin ki te frape m nan.

That is the boy who fell.

Sa se ti gason ki te tonbe a.

I want to buy the tallest tree.

I want to buy the tree that is the tallest.

Mwen vle achte pyebwa ki pi wo a.

Also check thess links:

http://sweetcoconuts.blogspot.com/2011/08/when-do-you-say-kisa-vs-kisa-ki.html

http://sweetcoconuts.blogspot.com/2011/08/so-there-are-two-ways-to-say-kisa-and.html

Ask me anything