The minimum API subscription period is 3 months. Thereafter you are free to cancel any time as long as you pay for the full month in which you cancel. E.g. if you cancel on the 1st of the month you will still need to pay for the whole of that month. However, if you cancel on say the 31 October then you do not pay for November.
Payment is usually made by credit/debit card or PayPal on recurring monthly subscription. Alternatively you can pay by bank transfer on presentation of an invoice but only for an annual subscription (not monthly).