Jenny, for the kind of Application you are planning to develop you need to take care of two things apart from Implementation:
1. Performance
2. Scalability
3. Database
For the Architecture there is no particular one that I can say, but I will suggest you to maybe use some CMS, maybe go for a Portal like Liferay. What that will help you with is:
1. A proven architecture. Its built on Java Platform and uses Spring Framework and Hibernate. Most importantly its a SOA.
2. Open source framework.
3. You can run it on
Tomcat
4. Security and Permissions are pretty good. You have features like Communities, Organizations, User groups.
5. You can build your own themes, there are a few already available.
For more you can visit : www.liferay.com
Apart from this you can take a look at this website, it explains youtube architecture :
http://highscalability.com/youtube-architecture
But for setting up something like that
you should consult some System Engineers.
Let me know if this helps.