I understand that there are quite a few kiwis across the board, so it could be worth explaining how my taxes work (and a few of these points will likely be similar for people in other countries as well).
Firstly, in NZ... As an individual (yes, I operate as an individual - only because you are required to be 18 in order to become the director of a company in NZ)... You are required to pay both tax, and ACC levies. You can find a list of current tax rates here.
Always keep a record of every single business-related transaction that you make (incoming or outgoing). Record the date, the currency, the amount, the amount in NZD, the exchange rate (as of that date), who you paid / were paid by and what the transation was for. There is a reason for this - the IRD may ask you for proof of expenditure, to verify that you are not ripping them off. Hopefully, this will also teach you to stop spending!
You are required to file an IR3 and an IR10A. Be sure to very carefully read the guide on the IRD website, each year - there is always a change, even if it is only slight.
If you purchase an asset worth more than $500, you cannot just write it off. You are only able to claim the depreciaiton. Depreciation rates vary heavily, depending on the asset that you purchase. You will be given two different rates - a Diminishing Value and a Straight Line. I recommend using the Diminishing Value for simplicity's sake. The current Diminishing Value on computers is 40%. If you were to purchase a $1000 computer, you would only be able to claim $400. The next year, you would claim $160 - and so on. If you were to sell that same computer for $400 (within the same year of having purchased it), you would be allowed to claim $600. If you were to sell the computer for $400 (the year after having purchased it), you would be able to claim a further... [original cost - sale price - what you've already claimed] = (in this case), $200. Use this tool to find depreciation rates.
As far as I know, you are able to pay tax via cheque, credit card or bank transfer. Personally, I pay using my WestPac bank account.
Any questions, if you're a kiwi and you're trying to sort out your taxes? Let me know, and I'll do my best to help you out.
P.S. My word is not law - do some research and back it up, before living by it.