What people worry about, honestly answered:

• "I'm not in shape."  Training is what gets you in shape. Every class scales and nobody's timing you.
• " I'll get hurt."  Beginners drill with controlled partners while coaches watch. You're not sparring on day one and you're not going home destroyed.
• "I'll be the worst one there." For a while, yes. Everybody was. The people two years ahead of you remember week one clearly, and they're the ones who look after new students.

What your first month looks like:

• Week one is learning how to move and how not to panic. By week four you'll recognize positions, know some escapes, and start understanding what's happening rather than just surviving it. Early progress in jiu jitsu is fast and obvious.

What you need:

• Shorts, a shirt, water. That's the whole list.