By playing the game, you should start unlocking items as you play. An important thought to consider is if you should spend money for items.
Should I buy/pre-order a game to get extra items in TF2?
Not sure about this. I pre-ordered CS:GO and got a special sniper that looks like the AWP. It's essentially a reskin, it's no better than the standard sniper rifle. I've seen it being sold on the market for about 3 bucks a piece, soo feel free to do so.
Should I trade items?
Sure! Just be smart when you trade! Don't give away items. Don't be afraid to ask for items or trades.
There have been times where I've been given crates for free, just ask.
Should I buy and sell items in the market?
Only if you are really going to use the items, or can sell them with a markup price. It's not too hard to get an extra 5 cents from micropurchases. If you are lucky, you can get much more profit. I've got as much as 50 cents from a single crate.
I got a new gun. Should I get rid of it or use it?
Try using it. If it's a good upgrade, keep it. If not, trade it, or craft it into something else.
Trade or craft?
Trading can be a good way to get exactly what you want. However, crafting items into others, such as scrap, could increase the value.