How can i make my seagate hard disk an usb hard

It is quite easy, do not worry.
You only need to bought and USB enclosure, here is a link to many products on ebay
http://search.ebay.com/search/search.dll?from=R40&_trksid=p0.m37.l1313&satitle=hard+drive+enclosure+usb&category0=

then you have to connect your hard drive inside the enclosure and the enclosure to your computer using the usb cable and that's it.
Just be carefull about the enclosure, I guess it is a Hard drive from a desktop, is in it? because some enclosure are for laptop's hard drive.
Also you need to know if your hard drive is SATA or IDE, IDE is the normal gray bus cable with fourty some pins, and the SATA it is a really small cable (I do not use technicims to not confuse you even more).

Also, there are some adapters from IDE/SATA to USB that you can buy in your local computer store (do not go to bestbuy, compusa, ... they only sell external hd), or also get it from ebay.
The only diference between the enclosure and the adapter is that the enclosure looks better :D, and the adapter is just a bunch of cables.

You mean you want to convert it in to a external USB portable hard drive..? if so you will have to purchase a enclosure. this has to be SATA supporting enclosure. there two kinds of enclosures, one for the notebook HDD size & the other for the desktop HDD. you will need to select the one which matches you HDD, also keep in mind that the desktop version need a external power source for it to function.

An internal and external hdd are virtually the same and can be altered according to the manner in which you wish to apply it very easily. here is a link to a site that sells hard drive enclosures and docks that will allow you to access your drive via usb.
http://www.tigerdirect.com/applications/Category/category_tlc.asp?CatId=2777&srkey=hard%20drive%20enclosure

Good Luck and pls rate this solution! :D