For beginners, building a new PC can be difficult, not just the building process, but the picking process as it can become very expensive fast, more so if you get something wrong, so I wanted to post something that may help new people.
So to start, before you even begin looking at parts is to plan the systems usage, What will the system be used for? Gaming, Video editing, Word processing or something else? this will define how much power you need in the system, word processing does not need the power you would need for gaming, so why over spend on a simple PC?
After you have picked a use case, decide how much you can spend, it maybe £500, it maybe £1500, only you can decide, have you got the ability to be flexible with your budget, can you stretch to that extra £50 for double performance? all questions that come up during the theory process.
After you have selected a budget for your plan, you need to begin looking at parts BUT do not go on the standard parts, I see a lot of builds with i7s for gaming, which is stupidly overpowered and not needed, there are always tricks you can pull off to reduce the power of games without loosing quality.
For gamers currently I recommend looking at i5 processors, more specifically Non-K for beginners, if you are struggling building overclocking should not come into the plan at this stage, now I am not calling you stupid, but your playing with expensive parts here, i5s can get expensive quick and blowing them and a motherboard get expensive dam quick.
I have been running a non-K i5 2500 for 3 years now, and never ever had an issue with it, it runs all games perfectly but games don't fully utilize the CPU these days.
So I recommend looking at something like the 4460 processor, its a brilliant little performer and not too expensive.
For graphics cards it can be a bit complicated, this is where the plan gets opened up wider and can make or break a build.
If your a gamer you want to throw as much money as you can physically can at this part (Within budget constraints of course) so if I had £500 to spend, half of it would be at the GPU.
Games take more advantage of GPU than CPUs, so more money will give you a better experience, more memory and faster chip overall.
For video editors, just offload work onto the GPU, its faster than CPU rendering.
You need to also learn to balance specs, so you can afford £250 on a GPU, not a problem, I can get a XFX 390 for that price, a top end card for cheap money, actually perfect there.
So I have 8GB GDDR5 memory that is insanely quick there, would run any game for the next good few years at 2k perfectly.
Of course if you want Nvidia cheapest 970 I can find is £257, which is £7+ over budget, now that is something I would find extra cash for or wait a bit longer, £7 is no massive amount these days, search the couch or something.
Motherboards are another place where people throw loads of money in but they shouldn't.
I have seen people request stupidly high motherboards, talking ROG level for non-K parts, pointless and wasted money if you want a red motherboard it can be sprayed (I advise against this though)
I got a high end motherboard for my i5 2500, I ended up with a G1 Sniper M3, but I was planning to crossfire then and I wanted some of the features that board offered me.
The other week I upgraded a friends motherboard, swapping him from AMD A6 to Intel I5, I got him a cheap Gigabyte board, absolutely perfect for him, he will never go into the BIOS, it also has all the features mine has and its about £80 cheaper.
Just get a cheap motherboard with Intel i5 non-K and have fun.
I wont go into overclocking boards unless asked, I am targeting beginners here not someone who can or will attempt overclocking.
Also all brands are good these days, all provide decent stability.
AMD FX8 series require specific boards due to the power draw, them 8 cores take a lot of power, so they need 8 pin motherboards (8Pin power delivery), but a quick google search will tell you the best boards for that chip, and they are not too expensive.
Memory is dirt cheap lately, 8GB in UK costs around £50 ish, just get 8GB 1600Mhz if you can afford it.
Power supplies, Simply put get a bronze certification PSU from a decent brand like Corsair, BeQuiet, CM, Seasonic etc, also get more than enough power, I normally double to wattage to power it so the PSU will only run at 50% load, this is not required as much these days although I still allow for around 15-20% overhead on PSU wattage for safety.
Cases are something where you can get a lot of band for your buck, I ended up with a Define R4 because I wanted it, but 200Rs do perfect for cheap builds, avoid these cheap £13 cases and spend at least £30 on it, you want a decent shell for your machine, and you want it to look nice and have good air flow, £30 is where all this starts to come into play, cable management holes are there and so are bottom mounted PSU slots, these are better because they don't drag in air from CPU coolers, and get their own fresh air supply.
Hard drives and SSDs come in last for budget to me, you don't need SSDs but they are good to have, OSes are fast enough these days to keep hard drives quick via software improvements like half shutdown on Windows, if you can afford get an SSD if you cant settle on the hard drive, it will serve well until you can purchase the SSD.
For HDD these days I recommend 1TB minimum, for SSDs, 120GB is minimum but 250+ is best, remember with SSDs the more storage you get the faster the drive is.
Also remember you need a copy of Windows, unless your using Linux, so remember to add a price of Windows in.
I wont go into peripherals here unless asked, they are something you can get later on, but I may add it later.
Also think about second hard parts, you can get some real bargains on eBay if you know where to look and what you are looking at.
Have I missed anything? This is a first draft mostly.
Hope it help in some way, also if I have put wrong information about something please tell me and I will correct it :)