Did you load jQuery in head section? Did you load it correctly?
<head>
<script src="scripts/jquery.js"></script>
...
</head>
This code assumes jquery.js is in scripts directory. (You can change file name if you like)
You can also use jQuery as hosted by Google:
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.min.js"></script>
...
</head>
Apparently, your web server is not configured to return jQuery-1.6.1.js on requesting /webProject/jquery-1.6.1.js. There may be numerous reasons for this, such as wrong file name, folder name, routing settings, etc. You need to create another question and describe your 404 in greater details (such as local file name, operation system, webserver name and settings).
Again, you can use jQuery as provided by Google (see above), however you still might want to find out why some local files don't get served on request.