These 2 particles は wa & が ga both do what in English is the subject, but は wa is greater in scope than が ga
は wa - the main topic particle of the conversation
ga - the subject particle of the sentence
わたしは クレイ です。
watashi wa kurei desu.
I am Clay.
[Clay is the topic and now this is known, it won't be repeated unless the topic changes]
ねこが すき です。
neko ga suki desu.
(I) like cats.
["cats" are actually the 'subject' here. Maybe this is easier to see "Cats are liked (by me). Note you could say "watashi wa neko ga suki desu." but it is unnecessary because we have already said "watashi wa" (The overall topic is already known)]
if both are in a sentence, the wa is first
the wa is written with a hiragana ha but pronounced as wa