Hi Dalo.
I must say easier said than done.
I spent many weeks reading the forums and picked up information and made many decisions based upon what the responses were to problems/advice people had. From all of this I found some consistent truths. The fact that I didn't have a clue what you were talking about sometimes sent me on other searches, trawling the internet for even more basic information e.g. two months ago I had no idea what a GPU or PSU was. I also watched several PC builds on youtube
This is what I have gathered but what to do with it or how to expand it is something else altogether.
1. Decide what sort of pc you are wanting based on what it will be doing
2. The pc case must be fit for purpose, a full size motherboard will not fit into a mini tower and if you are going to use it for gaming then it must be designed for maximum airflow and allow for the larger GPU's and heatsinks.
3. The motherboard must be capable of doing the job and preferably be able to cope with inevitable upgrades.
4. The CPU must be compatable with the motherboard.
5. The amount of RAM that you are advised to use by the programmes you intend to run, a) Can the motherboard support the amount of RAM you need, b) The operating system you use may dictate how much RAM it will recognise e.g. WIN 7 32bit will only recognise up to 4Gb of RAM so having more than that is a waste of money. WIN 7 64bit will recognise as much RAM as your motherboard will hold.
6. RAM comes in an incredible variety and is not interchangeable. You must be certain that you either buy the RAM you need all at once, for consistency or keep the packaging your RAM came in to be certain of getting RAM of the same specification.
7. Without enough power to run the PC it is all just an expensive paperweight. Get advice on the PSU you will require to run your system and then add 20%-25%. This allows for fluctuations in power and for future upgrades/overclocking.
8. Don't rush into it, ask advice and read the blessed instructions that comes with the stuff you buy (in my case it required several reads)
9. Update your drivers and your operating system.
10. I am 50 years of age with little computer experience and I just built a gaming pc for my son. Yes it scared the dickens out of me but I did it and it saved me cash by doing it and gave me a great sense of satisfaction but I don't have the backup of a warranty from a store bought pc. I do however have the support of some very clever people on this forum.
I now wish that I had recorded the build so that it could have been posted as an example. Some voice overs from you guys giving tips and explaining things would have been perfect.