a good website can be a complex beast or simple but efficient. It really depends on what you're trying to create.
Some basic rules should never be bent:
1) valid and standards compliant HTML and CSS (w3.org)
2) accessible content and navigation (w3.org/WAI/)
3) complete separation of content and format/style (
Separation of presentation and content - Wikipedia, the free encyclopedia )
4) Other keywords: Usability, Cross-platform compatibility, semantic markup
Don't ever look at what the big boys do (IBM, Amazon, Microsoft, EBay,etc) The big boys do not need great websites. They can afford to force you to use their mess of the website. Do what great web designers do, with the absolute emphasis on "web".
Then make it look great - but that's just me
Do you need a professional? Again it really depends on what you need. Is it your business site? Then I would always ask for professional help if you feel unsure you doing the right thing. Is it little personal portfolio or blog? Go ahead and play away as long as you're having fun
Hope this helps, if you need any assistance, give me a shout.
Cheers
Chris