Needing help installing plugins and theme

I need some help:

I try to install plugins and themes, but then I am asked about hostname, username and password.. Whatever I fill out there, it isnt right. Can anyone tell me:

1. What should I fill out at hostname
2. Which combination should I fill at username and password? Should that be of WP of my hostingcomplany or what?

Cant figure this out, please help..