How do I logon to the API?

Your API Login ID is the same as your username and password combination which you use to logon to our Control Panel. Along with your Login you also have to specify your company handle.

I would like to test the API first, is this possible?

Yes it is. You have to set <test> to 1 and you will connect with our test environment.  This is further explained in the API documentation. 

I get a UTF-8 error, what do I do wrong?

You are using special characters. The API only processes strings in UTF-8 format, likewise the output of the API is also in UTF-8. Use the utf8-encode function in PHP, this is further explained in the API documentation.