Hey man, let me tell you that creating your own homebrew can be SO rewarding - i have been doing it for 6 years now, and i have been playing D&D for 7 yrs. since i was 11. Crothian is right when he says do what feel right for your campaign. Start where you wish. If you want to choose races, classes, cultures and such, then do it that way, by all means!
For myself, it has always been about the maps - I was intrigued by Tolkiens maps in his books, and so i have always been creating exactly how i want my world to look, etc. Crunch time came last summer (of 04) and i really started to look at what races and classes i wanted in my campaign. from there, it has been a melting pot of ideas and such and i am haing great amounts of fun with it all
A few things to keep in mind:
Theme - what do you want your world to be? A few examples are D&D campaign setting, which actually pretty much cover the width and breathed of themes
Stone Age
Classical Greece and Rome [Bronze/Iron Age]
Ancient Orient [China, etc] [Oriental Adventures]
Ancient India
Dark Ages
Middle Eastern
Al-Qadim, Dark Sun (well, sorta)
Medieval [Birthright]
Renaissance [and or the Reformation] [
Forgotten Realms
"New World" [The Americas]
Feudal Japan [Oriental Adventures]
Industral Evolution
Modern Day [d20 modern]
Sci-Fi/Future [d20 Future/Star Wars d20]
Post Apocalyptic [d20 Apocalypse]
Also, you need
Bestiary - a bunch of monsters you want in your world
Classes and PrC's - what sort of "jobs" can your player have their charatcers take
Races, cultures and ethnicities - what races are featured in your campaign
And here are a few websites to get you started
The Mythopoets Manual - Religion and Campaign Creation
all about myths in a campaign and how they affect everything IYC
http://www.metamythos.net/world_building/mythopoet/religion.asp
Encyclopedia Mythica
a great web reasource for mythology, folklore and religion - great for races and monster ideas
http://www.pantheon.org/
Wikipedia
just a great, general reasource - you can look up anything in it
http://en.wikipedia.org/wiki/Main_Page
anywasy man, hope those things can help you - see you around the boards

;D