Is it ok to have I agree as the default option?

Yes, the "I agree box" being defaulted to off is legal issue not a UX one. You'll want to get your solution approved by your legal team; different teams have different standards and there's no one "most legal" way to do it, so that I have to leave to you.

1. How do you accept terms and conditions?
2. Do I need a checkbox for terms and conditions?

How do you accept terms and conditions?

“I agree to the terms and conditions as set out by the user agreement.” Or: “By clicking here, I state that I have read and understood the terms and conditions.” Using a clickwrap method may better protect a business because a user must take proactive measures to agree to terms and conditions.

Do I need a checkbox for terms and conditions?

Is an 'I Agree' checkbox a legal requirement? An "I Agree" checkbox is not a specific legal requirement. However, it will help you prove that you have obtained agreement and consent from your users, which can have strong legal implications.