I was charged before my 30 day trial period.

Hi, 
I got a message saying I could upgrade to a better plan. When I checked the options it clearly said that the plan I was choosing had a 30 day trial period and I wouldn't be charged any amount but until the end of that period. 
When I confirmed paypal as my payment option I got charged immediately and I now have a negative balance in my Paypal and bank account linked to it. 
How can I get a refund? I dont want to be charged in my bank for overdraft. 

Thanks.