That is a VERY vague question but I'll try to help...
First does your idea fulfill a need (be it market share that is not being attended by others or by a client who wants to give you money for doing this)? If there is no market then there is no income but if there is then you still have to approach it correctly. Your site will have to have certain goals as well.
Two - Do you have a roadmap of how your site will work? Will visitors be able to access the majority of the pages within a page or two of when they enter (be it any page as they can come to odd pages via search engines). Always good to draw a visual tree of your navigational layout.
Three - Do you know the proper technologies (or if you will be having others do this work for you, do they) that you will have to use for this project?
- As a sub question, if you want to go with Flash you need to ask yourself if the name of the site is highly associated and valued with something already so that it is likely many sites will quickly link to yours? If you you'll want to go with an ALL HTML (with perhaps very little Flash thrown in as for example just for the logo if you wish to animate it). If you go with Flash only links and the content from those pages will help search engines understand what you site is about as they are unable to go inside of a flash file.
Four - If you're making a portal your content will have to be both rich and dynamic. You'll have to setup a replicable template or system where adding and admining content won't be a pain in the ass. This will require serverside coding with very little doubt.
Since your question is so vague I just wanted to give you a base idea of what you're dealing with. Portal sites hve to be updated (sometimes daily) to keep traffic flowing and people interested.