Hi! First off, let me say that you are an awesome mom!
Second, I highly suggest that you build this yourself if you have the time/energy to put forth. It really isn't that hard, it's more or less just legos for adults. You can watch youtube videos on how you put everything together and how to install all of the necessary software. But if this seems like too much then try calling a local computer store and ask them if you brought them the parts would they be able to put it together for you?
Also, where do you live? This makes a difference when suggest parts and merchants to buy from. You don't have to be super specific, but at least the city you live in.
Here is the build I have put together:
CPU: Intel Core i5-4670 3.4GHz Quad-Core Processor ($219.99 @ Newegg)
Motherboard: Gigabyte GA-H87-D3H ATX LGA1150 Motherboard ($107.86 @ Newegg)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($63.75 @ Newegg)
Storage: Samsung 840 EVO 120GB 2.5" Solid State Disk ($99.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($69.99 @ Newegg)
Video Card: EVGA GeForce GTX 780 3GB Video Card ($494.70 @ Newegg)
Case: Corsair 300R ATX Mid Tower Case ($79.99 @ Newegg)
Power Supply: SeaSonic 620W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($59.99 @ Newegg)
Optical Drive: LG GH24NS95 DVD/CD Writer ($19.99 @ Newegg)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($99.99 @ Newegg)
Keyboard: Razer Blackwidow Ultimate 2013 Wired Gaming Keyboard ($127.99 @ Newegg)
Mouse: Razer DeathAdder 2013 Wired Optical Mouse ($51.99 @ Newegg)
Total: $1496.22
CPU: I chose a locked processor from the latest generation of CPU's from intel. This means that you will not be able to overclock the CPU, but if this is just for a 14yo casual gamer, I doubt that will make much of a difference to him.
Motherboard: I just chose a cheap mobo that supports your CPU. Gigabyte is a great brand, as well as Asus. I would limit my selection to those two brands, as they have the best handle on the Haswell chipset (I have chosen a Haswell based CPU for you).
RAM: I went with 8GB of ram, which is perfect for a gaming machine. When selecting RAM, you want to make sure that you are looking at a 2 module set (such as 2x4GB in this case) and also you want to look at the speed of the RAM. DDR3-1600 is pretty good for gaming, I wouldn't go any lower.
Storage: I have selected to storage drives for you. The first is an SSD for your Operating System and a few basic applications/games. An SSD will drastically reduce your boot-up times as well as your read/write speeds. It will make loading games and downloading files much faster. I also included a 1TB drive for mass storage. This drive is for everything else.
For the GPU I went with a GTX 780. This is a high end card that will run any current game at maximum settings. When building a gaming machine, the GPU is the work-horse. This is where you want to splurge the most. This card should not have to be upgraded for a few years.
Case: I just went with a cheap, minimalist look case. Honestly, this is where it comes down to personal preference. Make sure that you select an ATX Mid Tower case. A full tower will be huge and unnecessary for this build, and a micro-ATX case will not have enough space for the components. There are tons of cases out there, some with cool LED lighting, some optimized for silence, and other that just have a minimalist look. I prefer cases for Corsair because they usually have a nice sleek look, but Cooler Master is very popular as well as Fractal Design.
PSU: When selecting a PSU the brand is very important. You want one from Antec, Corsair, Seasonic, or XFX. I went with one that is semi-modular. All this means is that only the absolutely necessary cables are bound straight to the power supply, and the rest must be plugged in. This is great so you wont have to tie up a bunch of unused cables and stuff them in the case somewhere. It will help to make the build look a lot cleaner and tidier.
I chose Windows 7 just because I haven't used Windows 8. I have heard mixed reviews about it, so this is up to you.
The mouse and keyboard are both great for gaming. You want to stick with an optical mouse for gaming, and a mechanical keyboard for gaming. If you are bothered by the "clicky" noise of a mechanical keyboard, you can get a keyboard with different switches to make it quieter.
Sorry this isn't the most in depth review of the parts I chose for you. I hope this can be a jumping off point for you. If you are a first time builder with no tech experience, this forum and Youtube are your friends! You can learn just about everything you need to learn about part selection and assembly just by watching videos on youtube.
One last note, I highly recommend Carey Holzman's youtube channel if you want to watch in-depth videos on how to build a PC. He is absolutely amazing.
http://www.youtube.com/user/CareyHolzman?feature=g-subs-u
Hope this helps!