Find out which UART you're using.
Universal asynchronous receiver-transmitter - Wikipedia
When you know that, locate it's data sheet (google "nameofmyuart data sheet").
Eg.
http://www.ti.com/lit/ds/symlink/pc16550d.pdf
Locate the "register map" or programmer reference information.
Your're basically looking for
- the register that tells you characters are available
- the register where characters will be read from.
Find out how your UART is wired to your processor. Typically, your UART will appear at some hard-wired memory address.
Do you have an operating system on your processor?
Which compiler are you using?