IBM 5150  -  Basics, for 5150 beginners

About five seconds after power-on, there is a small flashing cursor in the top-left corner of the display.  You will not see a RAM count as you do with IBM's later computers.
This is a slow computer.  BE PATIENT.  After turn-on, it can be minutes before an error message is displayed.
An AT-class keyboard will not work.  More information at here.
Make sure that the keyboard gets plugged into the keyboard port, not the cassette port.   (If accidentally done, no damage results.)
The IBM supplied floppy controller and 5.25" floppy drive are double density.  They will not read high density 5.25" floppies, such as 1.2M (2SHD) ones.
There is no 'CMOS SETUP'.  All motherboard configuration is done via switches/jumpers.
There is no real-time clock (RTC) on the motherboard or on IBM-supplied expansion cards.  Some examples of third-party solutions are at here.
After turning off the 5150's power supply, wait at least 5 seconds if you are planning to turn it back on.  Any shorter; the 5150 may not start.
All four RAM banks on the 5150 motherboard are permanently enabled.  Therefore, you need to populate all four banks before adding a RAM card.
If the motherboard has the final BIOS revision of (10/27/82), then bugs in that BIOS result in the requirement for all four banks of motherboard RAM to be populated.
The 5150 motherboard does not have a keyboard controller chip.  Instead, it uses discrete components - see here.
The power-on self test (POST) of an IBM 5150 motherboard does not output POST codes.  Any numbers that you may see displayed by a POST card are not POST codes; they will be the result of something else.
Aged tantalum capacitors are known to explode.  That has happened to me many times, particularly when I have acquired something that has not been powered on in years.  If the motherboard, or expansion cards, are exposed to your face when you power them on, then consider wearing eye protection.  More information at here.