Assuming you mean for your business to receive a loan from someone else, go to Settings - Chart of Accounts, and create a Liability Account for the Loan you wish to track. You can set an opening balance if required when creating the account.
To receive a loan, go to Cash Accounts, Select the cash account the loan will go to, and Receive a payment to the cash account (Received In), and select the Loan account you created (Account) for the loan funds to be entered into.
Making a loan payment is basically the same, just select Spend Money instead of Receive money.
If you mean that your business will be making a loan to someone else, you can create the loan account as an Asset account rather than a Liability account, and follow basically the same procedure as above.