You can obtain the necessary connection values by logging into your Merchant account and clicking Account -> Settings. The LoginID and TransactionKey are then displayed in the Security Settings section.
LoginID: The API login Id associated with your payment gateway account. This property is used to authenticate that you are authorized to submit website transactions. Note that this value is not the same as the login Id that you use to log in to the Merchant Interface.
TransactionKey: The transaction key associated with your payment gateway account. This property is used to authenticate that you are authorized to submit website transactions.
UseSandbox: The Authorize.Net API to be used to process transactions. If you are using a production account, this property can be left blank. If you are using a developer test account, set this to 'TRUE'.
Below is an example connection string for Authorize.Net:
LoginID=myLoginID;TransactionKey=myTransactionKey