"A girl is swimming in a pond."
The simplest explanation i've seen is that が is usually used to emphasise the word/phrase before it. Andnは is used if to emphasise what comes after it.
Its best not to worry too much about it. You just get a feel for when to use them after hearing enough examples. You will hear certain recurring phrases always using が or は.
You may think of it as follows: - xy が simply marks the subject of the sentence. - xyは indeed puts some emphasis on it. It's like saying "as for the cat, it's in the kitchen", pointing out that it's not dog you're talking about. It also works, if you ask your friend たなかさん about his intentions, feelings, etc.. Consider this dialogue:
[たなかさん]: "かわ に あるきます"。 The first sentence points out that you talk about your plans for t o d a y. The second simply throws the ball back to Tanaka (literally "What about you?". --> Just in case you don't know yet, the perfectly normal way to address others during talking is to say his/her name. But あなた は should be fine also, at least if you don't know the name.) Generally speaking, if you can think of the sentence as "concerning xy,..." or "as for xy,..." or the like, then you should use は.It works for about 90 to 95%, since there are some minor exceptions.