You need to download iBroker app from your device, going to the Play Store or AppStore.
iBroker has versions for different brokers, so first you need to know which of them you will work with, and then Search it.
The available versions are here: http://www.ibroker.com/AvailableBrokers
You don't need to have an account with the broker to use the app, you can create a Demo account from the app itself.
To create a Demo Account, go to the Login screen or to the Help section and look for the "Create Demo Account" option. Once created, log in with your Demo Account, you will be able to simulate trading with full iBroker functionality. All trading taking place in this account is Demo, so no real money is made or lost.
If you'd like to open a Live account, you may fill out the "live account" form you may find the live login screen. Fill in the form and a broker representative will get in touch.