First off, does your computer actually have a USB3 port available?
If it doesn't, you might need to pick up a dedicated expansion card to plug into your motherboard—though, as those with a bit more technical experience know, you have to make sure your motherboard is actually compatible with that kind of upgrade. It’s pretty easy to figure out, though; just check which motherboard you're running and ask at a place like Best Buy (or just post the model here).
If your motherboard already supports USB3, then all you need to do is buy an external enclosure for your new 3.5" HDD that features a USB3 interface. Just keep in mind that most of these external enclosures come with their own dedicated power supply and wall adapter to plug into a standard outlet.
As for actually installing the HDD into the enclosure, it’s really not a headache. The kit usually includes all the necessary screws for assembly, along with the internal connector that plugs directly into the drive. You won't be able to get it wrong as long as you ensure the enclosure is specifically designed for the exact HDD you purchased... so just double-check the specs before buying.
Everything I've mentioned applies to getting a new external HDD enclosure... for example, something like this >
http://www.futura-it.com/product.asp...ID = US-GD-25633 which houses the HDD
P.S.—I was checking the warranty details for Seagate through a local distributor, and while their main site mentions a 12-month term as of early 2012, this specific link suggests a 2-year coverage. My advice? Always aim for the HDD with the longest possible warranty period...