Starting a wholesale LEGO business
You can buy LEGO in bulk at any official LEGO store. It is called the PAB (Pick-A-Brick) Wall. They will sell them to you buy the cup, or if you need really large quantities, you can also ask them for full boxes. Keep in mind that the selection changes seasonally, and you will not be the only seller who buys parts this way, so there will be competition.

Other than that, your only other option is to take advantage of sales at retail stores. Right before Christmas is an excellent time to stock up as many stores have great deals. Also, right after Christmas when stores are clearing out last year's stock to give room to new sets (LEGO always releases a whole range of new sets in January). 

LEGO does not sell parts in large bulks to individuals, unless you become an official LEGO Certified Professional. However these people get access to huge amounts of LEGO not because of reselling the pieces, but because they are creating art. In general LEGO does not like resellers and will not support them in any way. You will have to find your own resources/angle if you want to get into the business.