Don't. Buy the machine with the parts you need in it already. The only things you should upgrade are the hard disk and memory. Those are cheap, easy to find, plentiful, and easy to install.
Bigger things like CPUs and graphics cards are scarce and hard to find as just parts. They're also ridiculously expensive, and (at least, with graphics cards) may not even work on your motherboard.
Edit: Unless by "upgrade" you meant choose different parts on an online order form for the notebook - in that case, we need to know your budget and what computers you're considering.
I mean like buying it from the manufuacturer over the internet, im looking at about a 1500 budget. For example I was looking at some of the Asus G-series, the Alienware were a good example, m17 is now 999 base, and the m15x is 1099, while im not saying I am buying that, if had to chose what parts to upgrade from the site, what should I go for?