What dua is
Dua means supplication, speaking to Allah in your own words or with words from the Quran and Sunnah. The Prophet ﷺ taught that dua is worship (Sunan Abi Dawud 1479).
Quran 2:186 says that Allah is near, and that He answers the one who calls on Him when he calls. Quran 40:60 also invites us to call on Him with the promise of a response.
There is no fixed form. You can ask for forgiveness, health, guidance, provision or anything lawful, big or small. Nothing is too small to ask Allah for, and nothing is too large for Him.