In authorise debit orders

Today I had a look at my bank account and notice that there was 2 transactions (debits) on my bank account I then contacted my bank to find out which company debited my account they told me it was gym company,I never had any dealings or membership with this company how they got my banking details I don't know,the amount debited was R299 and R100 Can I please get information on this matter as it is a concern for me as my banking details are out there