Heres what you need to create a simple web-based sequenser:
Javascript + <audio> for playing samples + File API so user could use own samples + HTML for UI
-or-
Flash
But, i think it will be a difficult project for a novice
Search in google for "Soundation" to get an idea what it looks like.
If you want to write something but out of ideas, google "Timus Online Judge" and start solving problems.
Your interface is best made in HTML5 (w/JavaScript) or Flash (quicker to make apps with IMO) if you don't mind alienating the mobile crowd. For your backend you can use whatever you want, PHP/MySQL or a similar combination will do. For it to really work well you need to work out a good way to truly synthesize music on the fly (could be done by sending instrument data to server and then server does synthesis of sound waves) so you're not limited to samples.