For the button property called DisplayMode, put:
If(Or(IsBlank(txinFirstName),IsBlank(txinLastName)),DisplayMode.Disabled,DisplayMode.Edit)
Where "txinFirstName" is the name of your First Name text input control and "txinLastName" is the name of your Last Name text input control. This will make it so that the button appears greyed out and is not clickable until both text fields are filled out.
In the OnSelect property of your button, add the following to any operations happening there, remember to use a semicolon before or after depending on where you put it in your existing operations:
ClearCollect(colName,{FirstName:txinFirstName.Text, LastName:txinLastName.Text})
Again, where "txinFirstName" and "txinLastName" are the names of the text input controls.
Then on the screen where you're navigating to, add a text label with the text property:
"Hello, " & colName.FirstName & " " & colName.LastName & "."