how i can practise appian

Hello. You should be able to request a free trial environment, here: https://www.appian.com/platform/free-trial/. 

I recommend you log-in to academy.appian.com and enroll in our online Appian Developer learning path. There, you can access our "Developer Skills Practice Guide" and follow along with the steps to build an app in the free trial. You can access the full guide as part of the Welcome course, or you can follow along with the course series and complete each exercise as you go.