converting array from binary to decimal

dec is not initialized.

- '0' is inappropriate because a[j] is a bit (0 or 1), not a character code ('0' or '1').

Either j is not needed (you can use 31-i) or it is not calculated correctly (should start at 31 and work down to 0 while i starts at 0 and works up to 31, or j can be calculated from i in each iteration).

With those errors corrected, the program produces the desired output. However, there are a number of other issues regarding the correct declaration of main and certain aspects of style, so here is a new version addressing some of them:

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>


//  Declare main as "int main(void)" or "int main(int argc, char *argv[])".
int main(void)
{
    uint32_t f;
    int a[32] = { 0 };
    /*  Do not declare identifiers where you do not need them.  This avoids
        various errors that can occur where things are mistakenly used where
        they were not intended.

        i is used only in loops, so it is declared only inside those loops.

        dec is only needed after some other work, so it is declared later.

        j is not needed at all.
    */
    printf("Enter a value:");
    /*  Do not put a space between a function and the parentheses for its
        arguments.
    */
    scanf("%" SCNu32, &f);

    //  Use more spaces; do not crowd symbols together.
    for (int i=0; f>0; i++) //  Or "for (int i = 0; i > 0; i++)".
    /*  Indent a loop body more than the code it is in; do not put { and }
        further to the left and keep the loop body at the same indentation as
        the code it is in.
    */
    {
        a[i] = f%2;
        f = f/2;
    }
    printf("\nBinary number (LSB) is = ");
    for (int i=0; i<=31; i++)
        printf("%d", a[i]);
    printf("\n");

    /*  Put blank lines in transitions between code that finishes one task,
        like printing output, and code that starts another task, like
        converting to binary.
    */

    int dec = 0;    //  Declare dec here, just before it is needed.
    for (int i=31; i>=0; i--)
    {
        /*  Remove "- '0'" here.  a[j] is a bit (0 or 1), not a character code
            ('0' or '1').
            Do not use j.  This loop has a counter, i.  Using two counters for
            different things may have confused you.  While you want i to run
            from 0 to 31, you want j to run from 31 to 0.  You could use a
            separate j for this, but it is easily replaced by 31-i.
        */
        dec = dec + (1u << i) * a[31-i];
    }
    //  Include spaces in output, like after the colon, to avoid crowding.
    /*  Print a "\n" at the end of each line of output.  C is designed to use
        "\n" to end lines, not to start them, because "\n" causes output to be
        sent to interactive devices immediately instead of buffered.
    */
    printf("The decimal representation: %u\n", dec);

    return 0;
}