普通体の会話
Casual speech: dropped particles, 〜ちゃう・〜なきゃ・〜てるmachine translation
FormationPlain form + spoken-language contractions (〜てる・〜ちゃう・〜なきゃ…)
Casual conversation between friends and family uses the plain form instead of です・ます. Spoken Japanese also has a few habits of its own: the particles は・を are often dropped, and questions don't need か — just raise your intonation at the end of the sentence: 行く？ (going?).
Three contractions you'll hear every day: 〜ている → 〜てる (何してる？ — what are you doing?); 〜てしまう → 〜ちゃう and 〜でしまう → 〜じゃう (忘れちゃった — oops, I forgot); 〜なければ → 〜なきゃ (帰らなきゃ — gotta go home; the ならない part after it is usually dropped too). There's also 〜ておく → 〜とく.
Remember that this is casual speech for people you're close to. With people you've just met, superiors, and teachers, you still use です・ます as you've done since N5 — picking the wrong politeness level causes more offense than a wrong conjugation.
Examples
今、何してる？
What are you doing right now?
宿題を忘れちゃった。
Oops, I forgot my homework.
もう帰らなきゃ。
I have to get home.