は (WA) introduces the topic of the sentence (the main information, what we will talk about.)
私はアメリカ人です。
Watashi wa amerikajin desu.
I’m American.
WA identifies the topic of the sentence (the main information we'll focus on) when talking about a general, commonly shared truth.
子犬はかわいいです。
Koinu wa kawaii desu.
Puppies are cute.
Please note: the hiragana character は is usually pronounced HA. But when it's used as a particle, the character is pronounced WA!
2.
は (WA) is used to give more information about something that was previously talked about in the conversation.
木があります。その木は大きいです。
Ki ga arimasu. Sono ki wa ookii desu.
There’s a tree. The tree is big.
3.
In the structure "noun 1 + は (WA) + noun 2 + が (GA)," noun 1 is the topic (identified by は), and noun 2 (identified by が) is a piece of information that is a property of noun 1.
私のいとこは背が高いです。
Watashi no itoko wa se ga takai desu.
My cousin is tall.
4.
Used to compare two ideas. In that case, は (WA) can appear twice in the same sentence. Only one idea can be mentioned, but the comparison to the implied idea is still the same.
和食は好きですが、生魚は嫌いです。
Washoku wa suki desu ga, namazakana wa kirai desu.
I love Japanese cuisine, but I don’t like raw fish.
5.
The particle は (WA) can add emphasis on verbal structures such as "-te WA iru/imasu."