PCQ and Sozobon c
Shop your local bookstore for any tutorial books on C, I seem to recall one
that was written especially for people making the transition from Pascal to
C. Something like "C for Pascal Programmers", it was called. To answer
one of your questions, a C program always has several files open at once,
one of them is called "stdout", another is "stdin", and you can use
functions such as gets() to read characters from the keyboard, although
such lines need to be terminated by a newline, which shouldn't seem unusual
to a Pascal programmer. the libraries here are full of example code in C.
Take it slow, there's a lot to learn just in C itself, much less learning
to program the Amiga.