The basics
Start with "GDB executable"
The default GDB prompt is (gdb)
run the program with "start"
This sets a breakpoint on the "main" function.
Use "next" or "n" to step to the next line.
Use "step" or "s" to step into a function
Use "print" or "p" to print a variable
Use "list" or "l" to list source
GDB CLI
The basic idea to remember is that pressing Return repeats the last command.
GDB has only a limited programming language. In particular it doesn't have a way to put more than one command on a line. However you can use macros to run several commands with a single name, and this can be repeated using the Return or Enter key.