は (wa): the topic particle, explained
は is the first particle you meet in Japanese. It sets the topic of the sentence, the frame the rest sits in. Without it, you can't say "I am".
The pronunciation trap
First subtlety: the hiragana は normally reads "ha", but when used as a particle it's pronounced "wa". Every textbook writes は, but you say "wa". It's one of three particles with a special pronunciation to memorize from day one (the other two are を read "o" and へ read "e").
Its role: set the topic
A good mental gloss is "as for X…". The topic is what the sentence is about, what frames the rest. Nearly every introduction sentence starts with a は.
私は学生です。
わたし wa がくせい です。
I am a student.
Literally: "As for me, student I am." The は marks 私 (わたし) as topic.
今日は寒いです。
きょう wa さむい です。
Today, it's cold.
The topic is 今日 (きょう, today), the comment is 寒いです (さむい です, it's cold).
The implicit contrast
は also carries an implicit contrast. Saying "I like tea" with は can suggest "tea I like, but other things, maybe not". The further you progress, the more you'll feel this nuance.
お茶は好きです。
おちゃ wa すき です。
Tea, I like (it).
Implicitly hints at "...but coffee maybe not".