@matty5
The answer provided by @Mich_S is not entirely correct. You do not want to assign an opening balance to the liability account. That will be done when you assign the cost of the computer to the loan account when entering the bill.
To properly record this, create a liability account called "Computer Loan" or something similar. Also, make sure you have an appropriate fixed asset account called "Equipment" or "Computers". What you call the fixed asset account is up to you.
Then, when you enter the bill for the computer, assign the full cost of the computer (including tax) to the fixed asset account and enter the amount financed as a negative amount and assign that portion to the Computer Loan liability account. If you financed 100% of it, the bill will net to zero and show as paid. Then, when you make payments, assign the payment to the Computer Loan liability account. That will reduce the loan amount. If there is any interest, assign the interest portion of the payment to your interest expense account.