After Accounts Payable the next thing down your balance sheet might very well be HST Payable.
If you are HST registered this would be the amount of HST you collected from your customers, less the HST you paid on your purchases. For example if you collected $100 of HST from your customers but paid $20 in HST on your expenses you only have to remit $80 to the Canada Revenue Agency for that period.
How often you have to file your HST depends on your volume of sales. If your sales are under $1,500,000 a year you only have to remit annually. Between 1.5 million and 6 million the filing requirement is quarterly, and if you are 6 million + the requirement is monthly.
One of the most common mistakes I see is a client (or their bookkeeper) preparing the HST return a couple of weeks after the end of the period (say for example a period ending December 31st 2012). Then after they have sent off the HST return the client receives a bill from a supplier in late January that is originally dated December 29, 2012. In the accounting software this bill is entered on the December date. The HST included in this is then forgotten about when they prepare the next HST return. This is why it is important to do a reconciliation of the HST account with every return. For more on that please contact me.
When you call to register your company for HST, the CRA seems to default everyone to a monthly filer, when you would likely rather be an annual filer (less paper work is usually better). So be sure to specify that you want to be an annual filer, and make sure you have discussed with your accountant when your year end will be, since the CRA will ask you that too.
If you get registered as a monthly filer the only only way to switch to an annual filer is to file monthly HST returns until you are caught up and then switch, with can be a real pain!
Finally you must register once your taxable sales exceed $30,000 in any given year, but you may want to register early if you are going to spend a lot of money to get your business up and running. The reason is that HST is a net tax, so if you collect $0 (because you have no sales in the first year) but spend $260 in HST (on a $2,000 computer) the CRA will send you a cheque for $260! So register before you are going to make any major business purchases!