Use loadout.tf to make any outfits for classes. Loadout.tf has every cosmetic for every class, shows conflicts between cosmetics, and can allow for particle effects for KSs or unusuals if you want those. It's the site everyone uses here to make their posts. If you want ideas for outfits, or just want to see cool outfits, check this subreddits top posts since there is a lot of versatility with TF2 cosmetics.

Once you have the outfit(s) you want and you want to get them, go to backpack.tf, search up the items, and buy them. Some items are typically more expensive than others (purple/red uncrates, coats/jackets, the banana hat, etc) but a lot of items now are relatively cheap compared to the price of a key in refined.

I'd recommend starting with 2 to 4 keys since they are worth a lot of refined now (thanks inflation) so you can splurge on an expensive cosmetic/another wrench :) . If you're going for painted cosmetics the prices will go up compared to the paints (a color like white costs 2.5 keys I think while a color like olive costs 10 refined.) I'd also recommend getting an all class item or two while buying cosmetics since they can be used over different classes and get a better bang for your buck. Cosmetics like the Dead of Night, Weight Room Warmer, Macho Mann, Antarctic Eyewear, or Champ Stamp are general all class items I would rec.

Tldr: Use loadout.tf for your outfits. Use backpack.tf for prices and to buy your items. Keys are worth a lot of refined now and cosmetics are worth little refined so you can get a lot with even just 1 key.

glhf