and welcome to the Forum.
"How can I order F-security for only one pc?"
Windows or Mac versions? F-Secure Antivirus or the Internet Security suite?
To add to Blackcat post, assuming that you are using the Windows version, please purchase it online from here:
Internet Security suite:http://www.f-secure.com/en/web/home_global/internet-security?icid=848
Has somebody helped you? Say thanks by giving kudos. Has your issue been solved? Mark the post using "Accept As Solution" button to let others know.
To add to Fendy's post;
If you have a smartphone/tablet you could also try F-Secure Safe; http://www.f-secure.com/en/web/home_gb/safe#pricing
"Can you install it into my computer"
No, but just follow the simple instructions here.
When you have a valid subscription key, check the installation information here; http://www.f-secure.com/en/web/home_global/support/installers
If you are asking about installation instructions, I am supposing you have not installed the trial version; if this is the case, try before buying;
see for example http://www.f-secure.com/en/web/home_global/internet-security#trial