Well I'm here to help! Glad to hear that someone wants to get into the wonderful world of PC gaming.
Basically, for a gaming PC, it all comes down to a budget. What are you willing to spend, and what kind of time do you want to put into it.
There are many ways to go about this. You can build your own, by buying parts off a website like Newegg.
You can find a local PC store to build a PC for you.
Or you can go to a website like 'ibuypower' or 'cyberpowerPC'.
Personally, i enjoy putting in the time and effort to build the system myself. It gives the knowledge of the components in the system, and furthers my aptitude of fixing them myself.
As for main components, everything plays a pretty vital role.
The CPU is the processing unit that will determine the overall speed of your computer. It will delegate how fast you can complete operations such as file converting, or game loading speed. The good gaming ready processors can range from $135 all the way to $1,050. Intel and AMD are the two contenders here.
The HDD is obviously where all the data is stored, but the speed of the hard drive determines how fast the rest of the PC can read that data stored on the drive. Faster is always better.
There are 5400rpm drives, that i would NOT recommend for gaming.
The typical 7200rpm drives that I am using currently.
Super fast 10,000rpm driver that are usually special made for gaming.
Then Solid State Drives. These drives are very compact, and the fastest of the four. They are spendy, and come in small sizes.
I recommend brands such as Seagate and Western Digital.
RAM, in laymens terms, acts as the go between for the Hard Drives. In general, 4 to 6 GB of RAM is quite enough for gaming. It's the speed of the RAM that matters. As usual, the faster the RAM the more you pay. But for reference, I have 8GB of fast RAM that I paid $35 brand new. So it isn't a huge investment. G.Skill and Corsair are the best of the best.
The Graphics Card is where you will make the biggest difference. They range from $100 for the bare minimum to $1,500 dollars for the best money can buy. There are two choices, nVIDIA and AMD/ATI. Many people have a preference over which is better, (Sort of like the 360 vs PS3 war) and i prefer nVIDIA.
The Motherboard, or Mainboard is the platform of which all components are plugged in to. You have to be sure to find something that will work for your needs now AND for future upgrades. Brands such as EVGA, Asus and Gigabyte are great.
Then there is the MOST IMPORTANT part of them all. The power supply is the unit that powers everything in the system. If you go cheap on this unit, you could suffer the loss of EVERY component in your system. Spending an extra 30 bucks here to buy a QUALITY part could potentially save you hundreds if not thousands of dollars in the future.
I always recommend Corsair for the part, but I know the guys on here like parts by XFX and Seagate.
I'm sorry for the long post, but I hope your more educated because of it.
So the question now... is do you want to build your own in the future?