http://www.sheldonbrown.com/bbtaper.html

Your cranks need to be compatible with the bottom bracket, with regards to how steep the taper is on the square spindle ends and on the inside of the crank connection point. JIS vs. ISO. Also, your rear cog and the chainring up front need to agree about how far outboard of the centerline of the bike the chain will be, otherwise the chainline will be cocked. So, buy your parts in an order that will allow you to consider these things.