Once you're ready to close, your realtor (and real estate attorney, if you have one) will guide you through, step by step.
At the closing (sometimes called the settlement), the rest of the down payment will be due if it hasn't already been paid. Closing costs will also be due. You'll sign a number of documents including the mortgage, loan application, prepayment letter, disclosures, and other affidavits.²
If you don't understand something, just ask to have it explained. Your realtor and attorney should be willing to make sure you know exactly what you're signing.
After that, the house is yours! Congratulations on becoming a new homeowner.