Amazon cloud web application performance testing

I am struggling to get through performance testing which are amazon cloud based web application. we are going to launch our application in amazon cloud soon and we are trying to run the performance / load testing. Basically, i am trying to get result of our application service request / response time when 10 / 25/ 100 / 225 users using it simultaneously and network load test. What kind of approach do i need to follow since i was in to manual testing so far! is there any tool, where i can run and monitor my service request / response time.

Thanks in advance! Surya