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Umm, why?
If you're trying to hide stuff from parents/teachers/boss, no one here will or should help you. Otherwise, there's no reason to do so.
Posts like this make me want to help you. ("What? Why would you want privacy? What are you hiding?" - this attitude makes me shiver.)
Enable filevault on that account, admin account won't be able to see anything without your password or the master password.
Administrators can see your files. If someone has admin on your computer,
they can see your files. Unless you encrypt. However, you can't use filevault, because you must be an admin to set the master password, and if they have the master, they can get at your stuff.
However you can make a secure disk image and put your encrypted stuff inside of it. They'll be able to see that you have such a disk image (if they know what to look for), but no one in the world can see what's inside of it without the password.
Open Disk Utility is /Applications/Utilities. Make a New Image. Size it appropriately for your needs. Select Encryption - 128 or 256 it hardly matters. Chose your password. Finally, make the Image Format a sparsebundle. If you don't, Time Machine will back up the entire image every time you change even a single letter in a single file.
If your file name starts with "." as in .hiddenThing.sparsebundle, it won't show up in finder, even for an admin. The admin account still has the power to find the file by using the Terminal, if the administrator knows how to look for it (ls -A /some/folder will show him everything in the folder, even with a .) You can still access it using Terminal. To access the file issue the command "open /Path/To/.hiddenThing.sparsebundle".
Create the image in your home folder, or buried a little down in one of the folders in your home folder to prevent a casual inspection from finding it. When you want to access files in that image, double click it to mount it, and you will be prompted for your password.
Just remember, if you lose your password the data will be gone forever. That's the whole point, after all.