Yes and no. Yes, you can prevent your roommate from seeing your files, so long as they are inside your User folder. This works by default.
If you meant you added an additional 1TB hard drive (in addition to your already existing one) then you will have to play with some permissions to prevent him from being able to access your files on those drives. However, that does not RESERVE space specifically for you, or for him. That means he could fill up the whole 1TB of space if you're not using it.
You can assign Quota limits to him, but if he's an administrative user, he can just reset them. He can also change permissions on files to gain access to what you don't want them to have. So you have to set him up as a standard user. That will mean he can't install any software of his own, or make system changes of any kind.
You can change quota limits by right clicking on a drive and choosing properties, then going to the Quota tab, enabling Quota Management, then adding a quota management entry for his user account.
You could also partition the drive into two drives, and simply not give his account any permission to access your "drive" by right clicking on the drive and selecting properties, going to the security tab, and setting up a deny ACL for his account.
This is how it works in Windows 8 Pro.. not sure about Windows 8 standard.
