Как сказать на Русском? I'm excited

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Взволнован(а) has connotations of worry. It's derived from the word "волна" (a wave, like in physics), and used when you're not in a peaceful mind, your emotions oscillate. It's kinda like a tiny emotional roller coaster. I wouldn't use it when feeling strongly positive about something. But you can definitely use it when you expect something and have hopes it's gonna be good, but at the same time you worry that something can go wrong.
For example, Я взволнована своей предстоящей свадьбой. In general wedding is a happy event, but there can be worries about planning, being in spotlight, giving a speech or dancing in public, or simply about making such an important step, so you can be both excited and nervous - then that's a perfect word.
Or Я так взволнована - мне пришло письмо из университета! You've got a letter from a college, but haven't opened it yet. You're excited because you expect it to be an acceptance letter, but at the same time you're a bit worried it can be a rejection letter.

If you happily wait for something upcoming, and nothing bothers your mind, you can use these to express how you feel:
Моя любимая группа скоро выпустит новый альбом. Я в предвкушении! - My favourite band is about to release a new album. I'm so excited!
Жду не дождусь нашей поездки в Москву! - I can't wait for our trip to Moscow!