There are short answers, and very VERY long answers to that question.
The short answer? Identify trends in the market. I'm buying Halloween crates now just before October in the hope they'll rise for Halloween. Can do the same for Smissmass items in December, like the B.M.O.C, the Christmas hat.
The long answer? Scrap your current bookmark bar and add a ton of links to various TF2 trading websites for convenience. Mostly, use Backpack.TF, to learn item prices, see what your own inventory items are worth and your friends, and search the classifieds. The classifieds will show you who's paying the most/selling an item for the least at that time. You could also enter the raffles here to hopefully win some stuff for free.
The long answer is WAY longer than that, but I don't have two hours to help you unfortunately... if that answer isn't satisfactory, do let me know though!