To debug a program, start radare with the -d option. Note that you can attach to a running process by specifying its PID, or you can start a new program by specifying its name and parameters:
$ pidof mc
32220
$ r2 -d 32220
$ r2 -d /bin/ls
$ r2 -a arm -b 16 -d gdb://192.168.1.43:9090
...
In the second case, the debugger will fork and load the debugee ls program in memory.
It will pause its execution early in ld.so dynamic linker. As a result, you will not yet see the entrypoint or any shared libraries at this point.
You can override this behavior by setting another name for an entry breakpoint. To do this, add a radare command e dbg.bep=entry or e dbg.bep=main to your startup script, usually it is ~/.config/radare2/radare2rc.
Another way to continue until a specific address is by using the dcu command. Which means: "debug continue until" taking the address of the place to stop at. For example:
dcu main
Be warned that certain malware or other tricky programs can actually execute code before main() and thus you'll be unable to control them. (Like the program constructor or the tls initializers)
Below is a list of most common commands used with debugger:
> d? ; get help on debugger commands
