Your first Himalayan trek should teach you what you actually enjoy about trekking before you commit to a multi-day expedition. Start with a 2-3 day route with an established support network.
Acclimatisation matters more than fitness at altitude — build in rest days, ascend gradually, and treat any headache seriously rather than pushing through it. A local guide isn't just for navigation; they read weather and terrain in ways guidebooks can't.
Layer your clothing, break in your boots weeks before you leave, and pack out everything you pack in.