To fill in your company details, you can click the "Click to add your company data" text at the top of the screen, or...
In the right top of the screen, in the options menu click, the Company Profile
This is the Company Profile page, here you can add your details
This is the required fields, just the basics, whats needs to be in an invoice
And this is the optional details, you don't have to add it, only if you want.
When you are done, just click the orange Save button