Unless you're a richie-rich moneypants and can just buy all the equipment you need for the forthcoming loadout recommendations, it's important that you become acquainted with the Team Fortress 2 crafting system.
As you play the game, you'll find weapons and items randomly that will populate in your inventory and can be equipped to your class loadouts. Chances are, you're not likely to get what you want though, and that's where crafting comes in. You can break down items into resources that can be recompiled into something you find useful by following the in-game blueprints (found in the Crafting Menu).
By merging any two weapons from the same class, you'll get a Scrap Metal. Merge three of those and you'll get a Reclaimed Metal. Take a trio of those and tape 'em together and you'll have yourself a Refined Metal.
Once you've made the raw metals needed for your particular blueprint, you'll likely need a Class and/or Slot token. To build a specific Class Token merge any three weapons used by the desired class. Slot Tokens are crafted by merging three items of the same equipment class (Primary, Secondary, Melee). You can transmute a Class Token of one class into another by merging it with any weapon from the desired class. Slot Tokens can be changed by crafting them with a weapon of the desired equipment class.
Once you've got all the pieces needed (as dictated by the blueprint in the Crafting Menu), simply slap 'em all together and get what you want. Do this enough and you'll have enough equipment for customized loadouts in every class.
Do note: Some crafting blueprints may require you to merge Metal, Class Tokens, and/or Slot Tokens with other completed equipment. These can be rather tiresome to create, but often their potency is largely worth it.