I have the original iPad can I use a stylus pen to write in notes?

One thing to be aware of is that capacitive stylii have a rather broad tip. This makes them feel more like writing with a marker than a pen. (though the width of the line is controlled by the software.)

You will need an app that allows you to write. I like NoteShelf but there are dozens. You will also need a capacitive stylus, not one of the old Palm stylii. Amazon sells many, many varieties.