I don’t think the actual syllables matter too much unless you find them particularly abrasive. If you’re happy with something in English a popular choice is “I am”
You can try out different mantras during your meditation practice and see what vibes with you, and feels natural. If a mantra doesn't feel quite right, it's totally okay to switch and explore until you find the one that clicks with you. You can try "Om" or "So Ham" for start!
Happy meditating 🕉️🧘♀️