All you have to do is change the scenario you are using from,
Thread group = 1
ramp up = 0
Loop count= 100
to,
Thread group = 100
ramp up = 1 <or you can also leave it 0>
Loop count= 1
In the first scenario you are simply making 1 user request 100 times and hence each request only executes after the previous request completes. Think of it as,
for (i=0; i<100; i++) {
hit URL;
}
Whereas, in the second scenario you are telling JMeter to make hundred requests at the same time.