Unless you get really lucky, you're going to have to do a lot of trading. Usually by buying an item for a little less than it is worth and or selling it for a little more than it is worth. I am not a very experienced trader, so I would visit many trade forums and ask for tips on trading/good trade servers/etc. If you're patient, you can make a LOT of profit. I have seen many people go from a scrap metal to an unusual hat just from trading up little by little, but it takes a lot of time and effort.