beginning developing WebParts in visual studio 2008

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Hello,
I have begun developing WebParts,
My first webpart has this code:

using System;
using System.Collections.Generic;
using System.Text;
using System.Web;
using System.Web.UI.WebControls.WebParts;

namespace Sample.DisplayMessage
{
    [
     WebBrowsable(true),
     WebDescription("Displays a custom message"),
     WebDisplayName("Display Message"),
     Personalizable(PersonalizationScope.User)
    ]
    public class DisplayMessageWebPart : WebPart
    {
        private string customMessage = "Hello, world!";

        public string DisplayMessage
        {
            get { return customMessage; }
            set { customMessage = value; }
        }
        protected override void Render(System.Web.UI.HtmlTextWriter writer)
        {
            writer.Write(DisplayMessage);
        }
    }

}

Some errors appear and I don't understand how to rectify them.

Can you tell me what is wrong with this code and how to get the right version of the code?

Thank you.

UPDATE from OP:
For example the attribute WebBrowsable is not valid in this kind of declaration it tells me that it is valid in the declaration of indexer or property.