Purchase 10 bicycles for 5,000, paying 2,400 down and agreeing to pay the rest in 30 days.

How would I put this into a journal? What accounts would I use? Which accounts would be in Debit and Credit? And how much would the amounts be in each?

It depends on how the bicycles are going to be used. Are they being used by employees or are they being sold by your company. Also it will depend on what accounts your company have.

I will assume you are selling them so your accounts will most likely be:

Merchandise Inventory for the total purchase price
Cash for the amount of the down payment
Accounts Payable for the amount owed.