How do I know if I'm ready to become Muslim?

Mostly it comes down to belief and willingness. You're ready when you believe there is one God, accept Muhammad ﷺ as His messenger and are willing to keep learning. You don't need to be perfect, know Arabic or have everything figured out first.

The essentials

Becoming Muslim means saying the shahada, the declaration that there is no god but Allah and that Muhammad ﷺ is His messenger, and meaning it. Readiness means you believe those two statements and understand roughly what they mean.

When the angel Jibril (Gabriel) asked the Prophet ﷺ about Islam, he answered with testifying to these two truths, then prayer, zakat (charity), fasting Ramadan and pilgrimage for those able. About faith, he named belief in Allah, His angels, His books, His messengers, the Last Day and Allah's decree (Sahih Muslim 8). You don't need to master all of that. A basic understanding is enough to begin. See What are the six articles of faith in Islam?

What you don't need

Perfect knowledge. Learning continues for life, for lifelong Muslims too.
Fluent Arabic or a perfect prayer. You can learn how to pray after your shahada. See Do I need to know how to pray before I become Muslim?
A spotless record. Past mistakes are no barrier.
All your habits changed. You'll adjust step by step.
A special feeling or sign. Conviction matters more than a dramatic experience.

Signs you may be ready

You find yourself believing in one God and in the message of the Prophet ﷺ, not just finding it interesting.
You want to pray, learn and live by Islam, even though you know it will take effort.
Your questions are now about "how", not "whether".
Your decision has stayed steady for weeks, not just one emotional day.

Signs you may need a little more time

You're mainly doing it to please a partner, family or friend.
You still don't believe the core claims and would be saying the words only to fit in.
You feel rushed or pressured by anyone.

Taking time to learn is fine, but once you're convinced, scholars encourage you not to delay without good reason. See Is it okay to wait before taking my shahada?

If you feel ready

You can take your shahada at a mosque, with a friend or even privately, and then learn the practical steps afterwards. See How do I take my shahada?

Key points

Readiness means belief in the basics and a willingness to learn.
You don't need perfection, Arabic or fluent prayer.
Don't convert only to please someone, and don't feel rushed.
Once you believe, learning continues for life (Sahih Muslim 8 lists the basics).

Tip: Try saying the meaning of the shahada out loud in your own words. If it feels true, and not just nice, that's a good sign. If you hesitate, note why and ask a trusted imam or mentor about that exact point.