The actual process of transferring money from your bank account to another persons bank account online will depend on the banks website and how things are laid out.
But you should see an option in your online bank account to transfer money.
To transfer the money you'll need the persons bank account number and routing number as well as the persons name and the name of the bank.
Once you have all that information you just enter the information into a form on your banks website and then enter the amount you want to transfer to the persons bank account and then just submit the transfer.
Once you do submit the transfer it can take up to 5 business days for the money to show up in the other persons bank account.