I was watching Newegg's "How to Build a Computer" and on the boot drive part it said we should use the full hard drive for Windows. The boot drive that I am using is a 120GB SSD. Windows (I believe) takes up only 20-30GB. So do I need to use the whole thing? Or should I only use part of it? Do I even need to partition it?
A 120GB SSD will fill up fast. Considering most software need to install to the OS drive, yes use the entire drive. Some programs like Steam and Origin will allow a separate drive for games which is a great help, especially when an OS reformat is needed.
The reason for using a full hard drive and not partitioning it is over time is you 'use' the computer, that means on that C drive (where the OS installs, programs install, etc.) all your pictures, videos, DOC files etc. get placed there too. Eventually you use 'more then' 20-30GB, besides all the new patches, updates, and changes (from installing programs) to Windows itself that occurs.
I personally would NOT have only a SSD drive as the sole drive on a computer.
First they wear out faster if you don't "manage" them constantly then a regular HDD.
Second they improve the 'backend' of Windows working more 'snappy' and improve 'load screen times' but they DO NOT improve any program 'speed', that is relied on the RAM and CPU to process the data (and GPU to render it if necessary).
Third it is VERY easy to run out of space on such a small drive that when not 'paired' with a HDD (7200RPM 1TB for like less then $79? now) that has all the storage you need and isn't going to 'slow things down' waiting for it to 'load code' to the RAM when it asks for it.
Lastly adding a SSD requires a basic level to advanced level skill in computers, if you don't even understand (yet) the basic concepts of partition sizes and such, then this can blow back up in your face and you will lose all the Data 'suddenly' on the SSD as it is worn out quickly. Many things need to be 'tweaked' like turning off Virtual Memory, stopping Indexing, when installing a app purposefully got ot it's preferences and remap the TEMP/ LOG files to the secondary drive, etc.
If you google ADDING SSD tips and tricks, you can see the numerous lists of things one needs to do. If a SSD isn't installed by the manufacturer then adding it on your own is ON YOUR OWN... and you should have a high enough skill set to manage it. Just dont' want you to lose / waste money unnecessarily.