You've invested in the trading business by buying a ton of keys. You've found a trading server, and have traded those keys for metal, trying your hardest to sell them for their highest. So now you have an inventory full of metal and no idea what to do.
Well first, you have to get some product. Go to trade servers, see what's being sold. If you see strange weapons and hats going for under what they're worth [side note- for an idea of an item's worth, consult the spreadsheet: http://tf2spreadsheet.blogspot.com/], then buy them. Stock up on strange weapons, festive weapons, and hats. I'd stick to the lower end items for now, nothing over three refined unless you're sure you can resell it.
Once you've got a decent supply of strange weapons, and a few cool hats, begin buying all craftable hats. And I mean every single one. Start by offering one refined, but don't go higher than 1.33 refined. Now that you have an inventory full of items, it's time to start selling.