How do you open a file from user input?

I need to open a file that will be entered by the user and I don't know how to do this. I tried putting

Code:
ifp= fopen("%s", "r", &filename);

//also tried:

ifp= fopen(filename, "r");

Code:
printf("Enter the name of the file with the ticket data.\n");
scanf("%s", filename);

FILE *ifp;


so what is the command you use to open a file based on user input that was stored in a variable filename?