The most common mistake in putting together your own computer is not plugging things in correctly. Some comments on this:

If you have a socketed CPU, usually one corner of the chip is different (cut off slightly instead of square, or has a small mark on top), and the socket similarly has a marked corner. These marked corners go together. Most of the newer CPU types are also missing one or more pins on this corner, so that there is really only one way to plug them in, but this isn't guaranteed and I can't say I've seen all the different CPU variations out there.

For the floppy cable, the twisted end does not go to the motherboard! The twist of a section of the wires in this cable is used to distinguish between the first and second floppy drives. There are several different varieties of floppy cables, but all are essentially the same but just missing some features. The complete floppy cable has a total five connectors, as follows:

• A 34-pin (2x17) socket connector for the motherboard. Usually this is separated by the largest length of cable from the other connectors.
• A pin socket and a card-edge (slot) socket for the first floppy drive. You can only use one of these two connectors for a drive, and some cables may only have the pin socket here.
• After the twist in the cable, another pin socket and card-edge socket for the second floppy drive. You can only use one of these two connectors for a drive, and some cables may only have one of the two sockets here.

In almost all cases, 3½" floppy drives will have pin connectors and 5¼" drives have card-edge connectors. Other devices like floppy tapes may have either type of connector; use whatever you've got. If you're stuck without the right type of connector, there are adapters which can convert one type of connector to the other, but it may be just as cheap to buy a new cable with the right connector(s).

On the floppy/IDE/SCSI pin connectors, as stated above, always check to avoid plugging them in backwards by matching pin 1 of the cable with pin 1 of the sockets.

• Pin 1 of the cable is the colored edge, which may range anywhere from the entire insulation for one wire being colored red to barely-detectable red printing near the edge.
• Pin 1 of the sockets is usually labeled on the board or drive, or perhaps in the motherboard manual for the board connector. If you have a drive where you can't find the marking for pin 1, it's most often going to be the end of the pin connector closer to the power connector.
• Some cables have a tab and some sets of pins have a plastic sheath which allow the cable to go only one way, but if either is missing you can get it wrong.
• Be careful, if you have a set of pins without a sheath, not to plug in the cable offset by one pin in some direction; you'll usually be able to notice this because it won't go in easily, because you are bending pins to do this; normally these plug in easily. If you are sure you're plugging it in right but it still won't go, you may have a bent pin; inspect the pins and straighten any that don't form a nice, even grid.
• Most drives will not be damaged by a backward cable connection, but they won't work, and the drive activity LED will go on continuously to indicate a problem.
• The IDE cable for ATA/66, UDMA, etc. (faster IDE, like most hard drives made since 1998 or so) has 80 wires instead of 40 -- the extra wires are grounds to reduce interference between adjacent wires. This can be distinguished from floppy and older IDE cables because the ribbon cable quite clearly has a denser pattern of finer wires.
• SCSI has several different cable and plug styles, which apparently correspond to SCSI-II and SCSI-III, Wide varieties of both of these, etc., and the only kind I've ever worked with, a 50-pin connector like a floppy or IDE pin connector but longer, apparently isn't used much anymore, so I'm still going to leave this to somebody else.

Jumpers: As noted above, most IDE and SCSI drives will have jumpers to select master/slave status for IDE or SCSI ID number. These drive jumpers are usually smaller than the jumpers used all over your motherboard and on most cards, so be careful not to lose them!

Screws: You may find variations, but most standard cases only use two types of screws. One type is used for mounting the motherboard, cards, and the cover of the case, while another type, shorter with tighter threads, is used for drives. Don't use a normal screw here; you'll ruin the threading in the screw hole of your drive and possibly damage the internals of the drive if the screw is too long! These drive screws are like drive jumpers; they always seem to be the ones you lose and can't find enough of. If screws come from different sources, you may see different types of heads and different lengths of the normal screws, but in most cases they are interchangeable except for drive screws vs. normal screws.

Plugging in cards: Instead of a metal insert you have to twist and break out, some cases will have little metal panels that look like the parts attached to cards but with no card attached. (Some will have these in addition, for covering holes you once punched out but don't need any more). These screw in just like the cards do.

Power connectors: You will probably find 3 or 4 different types of power connectors coming from the power supply.

• Two of these types have four wires, with the middle two black (ground, or earth for those of you in the UK). You probably have one of the smaller variety of plug and three or more of the larger variety. These are the power connectors for the drives, and can only fit one way, though the larger variety is subtle: two corners on one long side of the plug are cut on a diagonal, while the other two are square, and the socket is similarly shaped. The smaller one is used by most 3½" floppy drives, and the larger one is used by almost everything else. These plugs usually offer some slight resistance when plugging them in properly, but pulling them out is never easy and sometimes requires considerable force; they will never fall out on their own or get pulled out accidentally, like the data cables can. If you don't have enough of these, you can buy a splitter which turns one of these into two.
• The motherboard power connector(s) have more than four wires. In the ATX-style cases/motherboards which nearly all new systems use today, there is just one big connector which can only plug in one way, but in the older style systems, common only a few years ago, there are two separate bundles of (different) wires going to two identical plugs, and the individual plugs are tabbed so you can't plug them in backwards, but be careful not to swap the two! On these systems, the black wires (ground/earth) go together in the middle; if you have colors in the middle and black on the outside you've got them wrong.
• There may be one or more small jumper-style connectors with 2 or 3 wires, and there are a couple possibilities for these; read your manual to find out what these are, and what (if anything) you can do with them.

If you get it all set up and turn it on and nothing happens, check all the power connections (monitor and PC, at the wall and case ends of the cords), check the monitor data cable's connection at the video card and also at the monitor (if it's not the kind built-in to the monitor), and check that all your cards and RAM and CPU are firmly seated in their slots. If RAM or the video card is not plugged in properly, you will usually get a beep code on the PC Speaker, assuming you have plugged that in properly. These codes vary with different BIOS manufacturers and may be documented in your motherboard manual, but most likely if you get beeps other than one beep to indicate a good boot, your problem is RAM or the video card.