Hello Ajmal,
A good starting point is to get familiar with the different widgets, options, and styling possibilities within mendix.
This article gives you a brief introduction :
https://www.mendix.com/evaluation-guide/app-lifecycle/develop/ux-multi-channel-apps/ui-design/
Also take a look into the learning paths :
https://academy.mendix.com/link/paths
And ofcourse the documentation :
https://docs.mendix.com/howto/front-end/atlas-ui/
Hope this gives you a kickstart,
Good luck !