i have some questions!
1) screwing the motherboard onto the casing
i am sure not all motherboards are compatible with all casings. how can i tell which one is compatible with which?
ATX/ITX/BTX standards define the locations of the mounting holes. Just make sure the size of your case can fit the board, e.g. if your case is a microATX case, don't try to fit a full ATX board into it. But a microATX board will fit fine into most standard ATX mid towers.
2) graphic card - vga card
it connects via the PCI Express slot on the motherboard right? are these made to standard specification sizes with every motherboard/casing because it has to pop out from the casing at the back? i saw a video on youtube that there is this small VGA card that needs to be slotted in as well, what does it do? give it VGA output?
The openings at the back of the case are standardized.
Ignoring older slot types, most PCI-Express cards will fit into any PCI-Express slot, even if the slot is shorter than the card. You just get potentially slower performance. But mostly only on the highest end cards and even then very minor differences.
Not sure what small VGA card you meant but in some very small casings, where you cannot insert a card upright, they may have a horizontal slot and will provide you with a riser that you insert into the motherboard so that the card will be installed horizontally.
Also in some rare cases, the board has onboard graphics/sound/network that isn't connected to an integrated output port and requires slotting in a riser card to provide the physical ports.
4) calculate how much power is needed
is it stated on each part that i purchase or do i have to estimate it?
Check reviews, estimates and calculate. In most general cases, a decent 350~400W is more than enough.
5) RAM compatibility with motherboard
i was told that there are 4 numbers on every RAM module. they need to correspond with the motherboard, if not they will never work.
Not really. You just need to make sure the board supports the type of RAM. e.g. don't try to fit a DDR2 ram into a DDR3 board. Server boards may require ECC ram but not likely an issue here.
You may have to install RAMs in pairs or threes for some boards.
6) parts compatibility
how can i tell if the parts are compatible in terms of specifications (not sizes)?
e.g.
this motherboard can support this type of ram
this motherboard can use this type of graphic card
Read the specs, google for information e.g. if you are planning to use board XYZ with card ABC, google if anybody has problems with XYZ and ABC. Of course make sure ABC can actually fit into XYZ first, you're not going to find people complaining about their IDE hdd being slow on their SATA only boards for example
Usually parts are quite well tested nowadays so the common stuff will work most of the time unless you're using very niche configs.
7) sound card
where can i place the sound card on the motherboard and how to tell if the motherboard can support it?
As previous, although I feel sound cards are a waste of money given the quality of onboard cards nowadays.
8) size compatibility
how do u guys estimate if the parts can fit into your casing?
Apart from expansion cards (graphics card, sound cards etc), all parts have standard sizes. For the cards, you only need to worry about the length, but this usually only applies to highest end graphics card which can be full length and some casings are too small to take them.