i do not where to start

Start with the free online documentation. From perlfaq4, How can I count the number of occurrences of a substring within a string?.

Adapting the FAQ example to vowels:

use strict;
use warnings;

while (<DATA>) {
    chomp;
    my $str = $_;
    my $count = ($_ =~ tr/aeiouAEIOU//);
    print "$str: $count\n";
}

__DATA__
Here I am.
djghsjkghk
It's me.
[download]

prints:

Here I am.: 4
djghsjkghk: 0
It's me.: 2
[download]