You will not be charged straight away - you will have a 7 day free trial. You will be charged after your 7 day trial ends. That is when your billing period will begin. For example, if you purchase a Monthly plan on May 2nd, you will be charged on May 9th and again on June 9th. The same goes for the Quarterly (you will be charged every 3 months) and Yearly plans.
Absolutely. Just log into the website and click on cancel. You will then not be charged again. You will still have an active membership until the end of the billing period.