Before you absolutely make plans to build a computer, though. You should:
- Ask parents if you can build one.
- Get help from them/have them watch you
- Consider the more difficult parts of building it, like installing the CPU fan, and putting all the plugs from the case on the right parts of the motherboard.
- Touch bare metal every once in a while to ground yourself, to prevent damaging a specific part.
You'll generally need a lot of patience, and having parents watching you would help a lot.
I think buying a pre-built is better. I can't see you needing all of that power.
What you can do is buy a pre-built computer, and install a video card so you can play most games decently. (Mostly depending on what card you buy)
It'll be cheaper to do it this way too. If you can find one with a decent processor in it, the most you'll need to do is add RAM and a video card.