Want to build a PC, but no idea what I'd be doing

Want to build a PC, but no idea what I'd be doing. Help?

Added (1). Someday, I'd like to build a PC that'll be used mostly for Dota 2, League of Legends, Minecraft, and Source Filmmaker. I have a friend who seems to be very good with computers, but I feel like I should know more stuff as well, but right now, all I'm familiar with is graphics cards. I'd like to get a GTX 750 Ti, but I don't know what kinds of other parts I should buy, since apparently, not every computer part will fit together or whatever.

Cameron 30.10.2015

Use: https://pcpartpicker.com/
PCPartPicker will auto check for incompatibilities
A few notes:
-All post 2005 GPUs and motherboards are compatible, since all manufacturers switched to PCIe
-PCIe is fully forwards and backwards compatible
-The most common motherboard sizes are ATX, mATX, and ITX
-A mATX board is simply a cut ATX board, hence mATX boards will fit in ATX cases
-If you don't intend on spending more than 650$-ish, then look at AMD processors, if you intend on spending 650$+, then consider Intel
-Stay with Seagate and Western Digital 7200RPM hard drives, avoid Hitachi and DeskStar
-An aftermarket cooler is a good idea of you're getting a CPU with a TDP over 50w
-Try to get 1866mhz or faster ram
-DDR3 and DDR4 are NOT compatible, (heck DDR3 won't even fit in a DDR4 slot and vice versa)
-Try to get an 80+ bronze or better power supply

Benjamin 31.10.2015