As a general rule: you can only protect your data against an enemy with skill the same skills that you have. Low skill = low protection, High skill = high protection (generally speaking).

Since you appear to be interested in your N900 (given the topic you've opened), I suggest (if at all possible) to invest some time in learning all you can about it. With time everything will look much easier than now.

One thing you could try is to put a file system in a file and then try to loop mount it. No encryption (for now), but at least you could manually mount a file system (onto a folder, e.g. /home/user/secret_folder), and then unmount it when it's not needed anymore.

You can start here: http://en.wikipedia.org/wiki/Loop_device