var Json: TJson;

with TJsonParser.Create do
try
Json := ParseUtf8File('c:\data.json');
try
ShowMessage(Json.ToString);
finally
if not Json.IsLiteral then
Json.Free;
end;
finally
Free;
end;

Create new json files:

var
MyArray: TJsonArray;
MyObject: TJsonObject;

with TJsonObject.Create do
try
SetNull('Null');
SetTrue('True');
SetFalse('False');
SetNumber('Number', 1.23);
SetString('String', 'hello');
SetDateTime('DateTime', Now);

MyArray := SetArray('Array', 3);
MyArray.SetString(0, 'First item');
MyArray.SetString(1, 'Second item');
MyArray.SetNumber(2, 3.21);

MyObject := SetObject('Object');
MyObject.SetString('First Member', 'Hello');
MyObject.SetString('Second Member', 'World');
MyObject.SetFalse('Third Member');
MyObject.SetTrue('Fourth Member');
MyObject.SetNumber('Other Member', 1.11);

ShowMessage(ToString);
ToUtf8File('c:\new.json');
finally
Free;
end;

with TJsonWriter.Create('c:\data.json') do
try
  BeginObject;
    WriteNull('Null');
    Write('True', True);
    Write('False', False);
    Write('Number', 1.23);
    Write('String', 'hello');
    Write('DateTime', Now, 0);

    BeginArray('Array');
      Write('First item');
      Write('Second item');
      Write(3.21);
    EndArray;

    BeginObject('Object');
      Write('First Member', 'Hello');
      Write('Second Member', 'World');
      Write('Third Member', False);
      Write('Fourth Member', True);
      Write('Other Member', 1.11);
    EndObject;
  EndObject;

  Check;
finally
  Free;
end;

Version 3.6
slash character is not escaped
Free method can be called on any TJson object, including shared TJsonLiteral