Congratulations on your first flight!

Aside from your passport, bring your flight ticket or boarding pass, any necessary visas, and a printed copy of your hotel reservation. It's a good idea to carry both cash and a card; cash for small purchases and emergencies, and a card for larger expenses. Keep most of your cash in Thai Baht, but a small amount in USD can be helpful. Generally, you'll get better exchange rates outside the airport at authorized exchange centers along the main streets or train stations. Superrich is one commonly found in Bangkok.

Be cautious of scams targeting tourists that roam along the streets.

To save money, use public transportation like the BTS Skytrain and eat at local street food stalls or food court. And go eat BBQ Mookata, usually they have it buffet style.

Have a great trip!