You'll have to download Jungleflasher and carefully read through the instructions in the Jungleflasher manual on how to flash your 360's drive.
You'll need a way to connect it to your computer, I recommend getting a X360 USB Pro v2, if you can't get that one, get a X360 USB Pro v1 and a CK3i Connectivity kit.
Also get a Probe v3, you might need it for drive key extraction.
You'll have to search in your local area for these bits of hardware.
I would suggest bringing it to a local modshop if you don't feel comfortable, it is worth the price.
Another suggestion would be an Xkey, it's a device that loads games off off a NTFS USB drive.
You'll have to purchase one and follow the instructions in this guide.
Again, if you don't know what you're doing or don't feel comfortable, bring it to a local modshop.