I am SalesForce beginner and trying to make an web application to manage projects.
I don't know where to start, what materials to make it.
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Sign up to join this communityI would suggest starting with Trailhead as it contains probably all basic things you need to know about Salesforce. You can also answer and practice coding with it as it will be connected to your developer account.
https://developer.salesforce.com/trailhead
You can also start making an app here: https://developer.salesforce.com/page/Quick_Tutorial_1:_Build_a_Cloud_App_in_5_Minutes
Apart from learning Apex and Visualforce you can go through this link. It is a very good article on Building Customer Facing Salesforce Apps. Regarding the project management app here is a source code available on how to create Gantt chart in Salesforce using Apex and VisualForce from Force.com Code Share. I hope this will be a good starting point for you.