Right here's a great place to start - you'll get all the info you ask for
1. Decide what you want the computer to be able to do.
2. Plan your budget.
3. Ask here for specific hardware recommendations - you'll get plenty.
4. Nip down to your hardware store and get a quality toolkit, or quality tools comprising at very least; 1 magnetic Philips screwdriver, 1 medium flat blade screwdriver, 1 pair longnose pliers, 1 wrist strap, 1 rubber mat.
5. Once budget and parts have been matched, buy all the components you need to get started.
6. Begin assembly;
Stand on mat and attach wriststrap.
Put Processor into motherboard, with thermal compound for the heatsink.
Put RAM into motherboard.
Put Motherboard into Case.
Attach floppy, hard disk and optical drives and graphics card.
Boot from floppy and install Operating System.
Add remaining peripheral cards one by one, and install drivers.
Add desired software piece by piece and test each one.
Enjoy your new PC
As I say, all specifics will be answered via this forum - there are a lot of very technically minded people here.
Building a PC from scratch can be one of the most rewarding (and possibly frustrating!) things you can do. Have fun! CitrixEngineer@yahoo.co.uk