Dear Rayyan,
Even I couldn't be able to find such a website which will provide facilities as you have mentioned.
But there are several sites are available that will take you accross to modeling in a brief like,
www.agile.com , www.uml.org, etc. beside this there are numerous book available that will illustrate
from requirement gathering --> analyis model of system --> design and coding process like, 1>"Applying UML & Patterns"
by Craig Larman , 2> Use case driven Object modeling by Doug Rosenberg, 3> Practical OOAD using UML 2.0 etc.
Another aspect is that, modeling is very tough whether you use OOAD or SSAD approach, because these skills are gained over
years struggle and experience in software development field. Therefore gain experience on some projects, and analyse
it within yourself, as you will overcome more and more difficulties you will find your questions are getting solved by yourself.
Regards,
Kumar Rohit.