It depends on the flow of the conversation. は and が is hard to practice in isolation as they are usually interchangeable. It depends on what’s being emphasized.
にほん の たべもの は きれい です。
As for Japanese food it is pretty.
にほん の たべもの が きれい です。
Japanese food is pretty.
In the first instance Japanese food is the focus, and pretty is an attribute you are bringing up about it.
In the second instance pretty is the focus and Japanese food is something that is pretty that you are now bringing up.
That said, I have difficulties with these as well, but I believe that’s accurate.