Invoice for payment

As this is our first invoice....so when i open my option for payment,online payment.I refill all,than i get one page where are asking me for company code,ussername....no idea  where i can get that information...As i clok option for for not registerd,but thay offer yust registerd in on line payment.so complicated really...can somebody explan how easy way to pay my bill...

Thank u Tanja