The new HDD has to be prepared in a spacific way before it wil work in the xbox. HeXEn is the tool you need to do that. It will prepare the new HDD and softmod it again, then it can also install a new dashboard and all the apps you will need.
Once you have that done find a program called "flashFXP" and install it to your computer. This program will allow you to transfer files to your xbox via the ethernet wire port on the back of the xbox.
if your going to be puting a TB in it you need to do some reading. HeXEn will prep it right but in order to get all the space you will need to use XBpartitioner. There is a search fuction at the top right hand side of this page. Search "1TB HDD upgrade" and read whatever you can find. There is far to much information for any of us to retype here. Reaserch, learn and then if you get hung up at any one point, come back here and we will help. But for the most part any question you could think to ask has already been asked. Use the search function first, then come back here if you cant find your answers.