A charter contract is a legally binding document, and the terms matter. A few key provisions are worth understanding before you put pen to paper.
The security deposit clause: understand exactly what conditions could result in your deposit being withheld. The cancellation policy: charter cancellation policies vary significantly — travel insurance that covers charter cancellation is worth considering.
My general advice: read the contract. Ask questions about anything you do not understand. A reputable charter company will be happy to explain their terms. If they are not, that tells you something important.