Sorry, you really can't assemble a laptop. It is possible, but because so many parts are needed from OEMS, like GFX cards, its nearly impossible and would cost a fortune. Also, explain what kind of laptop you need, i.e. gaming or web browsing.
There are some barebone / whitebox laptop kits available.
It's technically possible to 'build your own'... but it's going to be more expensive if you're not buying lots of several hundred kits, HDDs, optical drives, etc.