I've built numerous computers for myself and friends and its really quite simple. Here are some tips that I'd suggest.
1) Don't be cheap when buying your power supply. Add-in cards, especially some graphics cards, can be power hungry beasts. Buy a high power quality power supply.
2) Buy as much ram as you can afford. It's cheap and you will see a difference in performance, especially if you do any video rendering or resource hungry applications like Photoshop.
3) Spend the money for a good solid state disk drive. Boot time and any drive intensive task will be much faster with a solid state drive.
3) Personally I stay away from the largest drives. Unless you have a lot of photos or other large files I'd stay with a smaller D: drive, say 2TB as thier MTBF will be much better than the bleeding edge high capacity drives.
In case the above is a little confusing when I build a computer for myself I put at least two drives and often three in my machine. I buy a smaller SSD, 128GB or 256GB, for the boot drive, C:, and load the OS and programs on it. I then put in a second high capacity drive for storing all my files on, usually 1 or 2 TB. When I was sing my computer for word, $$$, I would also put in a third drive that was used exclusively to do nightly backups on. This third drive wont replace the need for offsite backups but it can save you if your lazy like me and don't make regular backups to some form of removable media.
4) Make sure you have lots of USB3 ports.
5) Make sure that your video cards will support at least three monitors. You may never use all three but they are nice to have just in case.
6) While we are on monitors I prefer to use three smaller monitors as opposed to one large monitor. I've had three monitors on my computer for a couple of years and I might go back to two for space reasons but I'll never go back to one.
7) If you have the room buy a large tower case. You can get a larger power supply and the air circulation will be better than the smaller cases.
8) One last thing. Consider replacing the fans in your case with fans that a) use ball bearings and b) have a rheostat for adjusting the fan speed. This may seem like a minor point but it will make your computer much quieter as you can adjust the fan speed and they wont start squealing after running for awhile.
9) One more last thing. Buy the fastest processor you can afford. For me it's usually the next to the fastest processor sold by Intel. (I live a few miles from a major Intel facility so I have to support my local economy but there are other good manufacturers out there.) I buy the second fastest because I'm kinda cheap and can never justify the large jump in price to get the fastest CPU.
Hope this helps.
BTW, @
oldbuddy is on track. If you can find someplace to build you a computer for $25-50 and they stand behind their work its a good investment.