"There is a kitchen."
が is for something that "there is/are". But there's much more to its usage than this.
があります。(There are vegetables.)
が is also for a subject, always. It's the subject marker.
わたしがAVAX3Mです。(I am AVAX3M.)
いすがおおきいです。/ 椅子が大きいです。(The room is big.)
は is for a
subject topic, not subject, well it can be, but not necessarily, not always. It's the topic marker (more info below..).
はがくせいです。(I'm a student.)
The "I" is the topic, which also happens to be subject in the sentence.
いまはがくせいです。/ 今は学生です。(I'm a student now.)
Here, the topic is "now", but the subject can be implied to be still "I", and not "now".
を is for an object receiving an action.
をたべます。(I eat fish.)
The fish is the object, to be eaten.
Sorry if I couldn't explain well since I'm still learning them too so as always everyone's welcome to question and correct.
I made a few corrections above and here are some explanations from PuniPuni Japan article. It's a great read, please do try!
★ Use ga (が) when information about a subject is important or situationally new to the listener and/or the speaker.
★ Once the subject has been mentioned, wa (は) is used to refer to the same subject in sentences following (it becomes the topic of the sentence.)
So for example, if I've been talking about a chair..
は大きいです。- The chair is big.
は赤いです。- The chair is red.
Then suddenly, I noticed a room..
が青いです。- The room is blue.
The subject is now "room", a new subject. Hence, the subject marker が. If I am to continue talking about that room, then..
は大きいです。- The room is big.
..the topic marker は is used in place of the subject marker が.
Japanese from Zero is great! Especially his grammar videos: https://www.youtube.com/watch?v=RsrakMT1h2glist=PLOcym2c7xnBwRNLWXOAtlN-9dbLkctxV-index=1
And his in 5 videos: https://www.youtube.com/watch?v=8dB2gWNtNOU
I found this wiki page that helped me understand は and が.