WRONG
She spent three hours preparing to the court hearing.

RIGHT
She spent three hours preparing for the court hearing.

This is a very common mistake. But “prepare” is not always followed by “for”. Sometimes “to” is correct.

The rule is as follows:

Prepare for + noun (where “for” is a preposition)

Prepare to + verb (where “to” is an infinitive marker)

Examples:

Prepare for + noun

We will need some time to prepare for the meeting.

He clearly had not prepared for the interview – he couldn’t give decent answers to even the most basic questions.

My son is having extra Maths lessons to prepare for the exam.

You can also prepare somebody for something, e.g. Have you prepared your witness for the hearing?

Prepare to + verb

The instructor opened the aeroplane door and gave my parachute one final check. “Prepare to jump”, he said.

There’s going to be a fire drill in ten minutes. Prepare to evacuate the building.

The above examples have a literal meaning – “prepare” means “prepare”. But very often “prepare to + verb” has the idiomatic meaning “to be willing to do something”, as in the following examples:

He said he was prepared to listen to all opinions, but he didn’t care about what I had to say.

I am prepared to do anything to help that is within the law. But I’m not prepared to do anything illegal.

We are not prepared to pay any further expenses.