To enter data into a list with PowerApps, the data source must first exist somewhere. PowerApps does not create data sources, but links to them. There are a variety of data sources you can connect to.
The easiest way to do this is to use one of the creation templates with PowerApps, connect to a data source and let it build the basic screens to view, edit or add data for you.
Create a blank spreadsheet.
Create three column headers by typing in Title, Author, Description in three separate cells next to each other.
Select the three cells and push ctrl+T to create a table (this is what powerapps will connect to in your spreadsheet)
Make sure the tick-box "My Table has Headers" is ticked, click OK and save your spreadsheet on OneDrive for Business
Sign into PowerApps.com (preferably with the same account you used to save the spreadsheet to OD4B).
Select "Create New App" and select the OneDrive for Business template.
Select your spreadsheet, then select the Table you created (should be Table1 if you didn't rename it)
You're done. PowerApps will connect to your data and set up some screens that will allow you to view, edit, delete or add data to your list. If you run the app and hit the "+" sign, it will take to you the add item screen and you can add away.
If you have an existing app with an existing data source, then just insert an Edit Form on a Screen and link it to your data.
With the form selected check the options on the right of the screen and click the database icon to select your data source.
Check the name of the Edit form, (usually an iteration of 'EditForm1', feel free to rename it or leave it as it) then create a button or icon to upload the data and set it's OnSelect value to SubmitForm(youreditformname)
When you enter values into the fields and click the button, it will update your source with a new row/item.